The term Seeker refers to Decepticon jet troopers who all share the same body style. That is, the Decepticons in Generation One that looked like Starscream, but in different colors or with minor variations in wing and head shapes, and also the Decepticons in later franchises such as Armada where similar "families" of jets appear.
The word "Seeker" hovered a long time in a strange gray area between official and fan-coined terms. It seems to have originated in extremely obscure official or semi-official writing, but somehow became widely used among fans. In 2002, the term appeared in dialog from the first issue of The War Within from Dreamwave Productions, making it truly official after years of controversy. It has subsequently appeared in many other places such as on toy packaging, books, and in other stories.
- Japanese name: Jetron
Generation One Seekers
In Generation One, there were six Seekers originally given names and characterization: Starscream, Skywarp, Thundercracker, Thrust, Dirge, and Ramjet. The first three, the season one Seekers, had animation designs done primarily by an unknown designer in Japan.
The last three Generation One Seekers are usually designated as Conehead Seekers by fans for their distinctive transformation that leaves the jet nosecone pointing up. The animation models for the Conehead Seekers adhered more closely to the toys' proportions and were designed by Floro Dery.
And all the rest
In the Generation One cartoon there were large numbers of generic, unnamed Seekers in addition to the named characters. They came in a wide variety of colors, from extra duplicates of Starscream or one of the other named guys, to original and distinct looks all their own. They seem to have formed the bulk of the Decepticon forces on Cybertron, as well as among Megatron's initial troops on Earth. They gradually became less common, perhaps as a result of casualties... or the cartoon's production team becoming more careful, or more-likely having more "actual" Decepticons to work with as the toy line expanded. (Extra Reflector units and alternate color cassettes were also used along with these Seekers, early on, to fill out the comparatively thin Decepticon ranks.)
These unsung Seekers would be recognized years later in Heroes of Cybertron toy form as the Air Warriors (all using the common lavender/white color scheme). Soon afterwards, the Dreamwave Generation One comic series introduced an army of similarly-colored Seeker drones.
Around the same time, another background Seeker was also given a new life through toys and Dreamwave comics: Sunstorm. This yellow-orange Seeker was a nameless background character in the first episode of the Transformers cartoon, until e-Hobby turned him into an exclusive toy (available with an Autobot also based on a first-episode cameo). Now Hasbro can't get enough of him. Another generic Seeker was given attention: Acid Storm. Like Sunstorm before him, he was based on one of the background Seekers from the original cartoon, in this case the episode "Divide and Conquer" (see below).
Unnamed Seekers with their own distinct color schemes appear in the following episodes as noted below:
- "More Than Meets the Eye, Part 1": - In the "return to Iacon" scene on Cybertron, five to seven Seekers appear (wide shots depict a group of five, but the close-ups give us seven distinct color schemes.) The first three are (as far as colors are concerned) Skywarp, Starscream (with a deeper voice), and Thundercracker. In a following shot, we see a lavender Seeker with a flamethrower pack, a medium-blue Seeker with white trim, holding his arm gun like a rifle, a yellow/orange Seeker who would become Sunstorm... and lurking in the back, a very dark-blue Seeker with white trim barely visible, but there if you look.
- "More Than Meets the Eye, Part 2" and "3" - The motherload of unnamed Seekers. Especially if you like variations of blue and lavender. Crowds of them appear at the start of the final battle. At least six different Seekers of various shades and decos of blue and at least or four of various shades and decos of lavender (two with black tail fins in jet mode, two with white). Also, a blue variant (seen in the picture above) missing his wings and shoulder vents appears in "Part 3". (The infamous "Lady in Purple" a piece of well known fanon, for possibly being female due to weak line strength in the art also appears here)
- "Divide and Conquer" - Avert your eyes! One deep blue, one eye-burning bright green, and one garishly bright yellow. This trio is often referred to by fans as "the Rainmakers" because they started an acid rain shower to irritate a group of Autobots. (Note: For one brief part of one scene, a red Rainmaker jet also appears with these guys. Make of that what you will.) The green jet in this grouping was made into the Universe Acid Storm character, whose name was initially supposed to be "Rainmaker," then "Acid Rain."
- "The Ultimate Doom, Part 3" - Two blue variants, and one really nice looking pine-green Seeker appears.
- "Starscream's Brigade" - One conehead-style Seeker with sweet black and dark grey colors appears in robot mode next to Megatron when he attacks Autobot Headquarters. This seeker looks a lot like Thrust.
- "Five Faces of Darkness" - One all-gray Seeker, one with Onslaught's coloration, and one with Silverbolt's. There's also an interesting Conehead variant. (All of which appear in the main image above.)
- "Dark Awakening" - One season 1-style Seeker with Dirge's colors appears in jet mode next to the actual Dirge.
- Puffy stickers - A sheet of Transformer puffy stickers featured a rare generic with a Starscream-based color scheme. He is pictured above as well.
Unicron Trilogy Seekers
According to one minor source in Armada, when any Transformer converts to a jet, he is called a "seeker." But the closest specific characters to the Generation One Seekers are this universe's own Starscream/Thundercracker/Skywarp trio. All three even used the same toy mold in their first release, although Skywarp was heavily retooled from the others (including getting an entirely new-mold Mini-Con partner). In scenes of Armada when the Decepticons return to Cybertron (and in some flashbacks), several Decepticons in military assembly or attack scenes are made to resemble Starscream's Cybertronian or Earth character models, minus a few details (usually his wings or shoulder-mounted intake/null ray cannons). While new Decepticon-allied jet characters named Thrust and Ramjet were released in Armada, they each had their own unique molds. However, whereas Ramjet was simply the Mini-Con partner of Tidal Wave, Thrust was a bulk who even had a VTOL engine in jet mode, a conehead and vertically-oriented wings in robot mode. Although Thrust's original toy was green, redecos looked a bit more like his G1 incarnation.
In the post-Armada parallel Universe franchise, a fourth Armada seeker was created, Ramjet, using Skywarp's unique retooling.
Though only Starscream appeared in the follow-up franchise, Energon, several Seekers appeared in the final portion of the Unicron Trilogy, Cybertron. Starscream was back in a form heavily inspired by The War Within Starscream design (itself based on the "tetrajet" (see below)), while Thundercracker received a completely different body. A Skywarp was also released as a redeco of Thundercracker.
Further, the Legends of Cybertron Starscream toy, a tiny, simplified version of his main toy, was redecoed three more times: Skywarp as a giveaway at ComicCon 2005, Ramjet as a giveaway at BotCon 2005, and Sunstorm as part of the last wave of the LOC line at retail.
In the second issue IDW's The Reign of Starscream series, a sequel to the 2007 live-action movie, Starscream and Thundercracker are referred to as Seekers for the first time. However, according to writer Chris Mowry, in this universe the term Seeker has more to do with the skill sets that they possess than with the body-type that the two thus far identified as such happen to share. He describes the Seekers as a crack unit of Decepticons under Starscream's command akin to the Navy SEALs, possessing superior tracking and detection skills and a high-level of firepower, who can "just get things done better than anyone else".  Knowing Starscream, he reminds everyone of this fact every chance he gets.
Revenge of the Fallen
In fourth issue Defiance, the term "Seeker" is used to describe the early Cybertronians spawned by the All Spark who are capable of space flight. According to The Fallen, these Seekers are responsible for finding suns across the galaxy and leading the Cybertronian to these suns in order to collect Energon.
In Revenge of the Fallen, its revealed that many of these Seekers are on Earth and at least several are in the United States. According to the novelization, they are both Decepticons and Autobots. They are the only remaining Cybertronians who can read the Language of the Primes. They are sent to find both the Matrix of Leadership and the Solar Harvester. One of them was an ex-Decepticon mercenary who was apparently a major Seeker, judging from Wheelie's comment. He was very old and ended up in stasis lock in the Air and Space Museum. He was awakened from his stasis by Sam Witwicky with an Allspark shard. Jetfire sacrifices himself after being mortally wounded by Scorponok to give the revived Optimus Prime the power he needed to defeat the Fallen.
In the Transformers: Revenge of the Fallen video game, a Seeker is a Decepticon jet that fires a slow-rate machine gun as his primary weapon and a continuous stream of missles as his secondary weapon, and has the special ability to teleport. Sound familiar?
War for Cybertron
A Seeker is a type of drone used by the Decepticons. They are more of a hindrance than enemies — Of course, enemies are hindrances too. The Starscream/Skywarp/Thundercracker chassis is referred to as an "Energon Seeker".
The term "Seeker" may have referred to flight capable Cybertronians or a military designation for flight capable soldiers.
During the Civil War, most of the Seekers joined the Decepticons, but some joined the Autobots. The Autobot Seekers were known as Arialbots.
The most obvious alternate name for these Transformers—and the one generally used by pack-in cross-sell catalogs—is Decepticon Planes. While usually clear enough from context, this term has the weakness that there are many Decepticon planes who are not "Seekers". Also, the term is rarely, if ever, used outside of toy-specific contexts.
The same Transformers are also sometimes referred to as tetrajets, which makes reference to their Cybertronian forms as seen in the cartoon episode "More Than Meets the Eye" and a handful of other episodes set on Cybertron. In these forms, their vehicular modes are shaped somewhat like tetrahedra, or "triangular pyramids" (that is, a pyramid with a triangular base). The tetrajets themselves appear to have been based on designs for the never-built WWII-era German Lippisch P.13a delta-winged fighter.
For a time, the term Skyraider saw increasing popularity. It originates from European Generation 2 marketing. When Starscream and Ramjet were released in this line, their packaging referred to them as Skyraiders. The UK Generation 2 comic also featured some character profiles (much like the old Transformers Universe comics), and those profiles for Starscream and Ramjet used the word as well. It is a relatively small leap to extend usage to all similarly-built Decepticons. The push to use "Skyraider" was largely founded on the idea of using an official term instead of one that was, seemingly, coined and used only by fans. When Simon Furman made use of "Seeker" in The War Within, however, this primary motivation was lost. Despite this the term had somewhat of a revival at Botcon 2009, when Skyquake's techspech stated that he designed "the first Skyraider frame."
The first UK cross-sell catalog referred to the group as strike planes. 
In Japan, they received the sub group designation known as Jetrons.
Origin of term "Seeker"
Although the most well-known name for these Decepticons, the wide use of the term "Seeker" is still somewhat mysterious.
The only known, documented use of the term is from the 1984 J.C. Penney holiday catalog (sometimes called a "wishbook"). On the page which shows the Christmas season's available Transformers, the following entry is found: "Decepticon Silver Plane. Airplane with sensational F-15 styling scours the countryside searching for Autobots. When they're found, the Seekers set out to destroy them. Transforms to an exciting robot with hi-tech weapons and Decepticon logo." A photo of Starscream and Thundercracker is displayed.
Also on the same page in an ad for Soundwave, he's said to send out messages to the Seekers and other Decepticons.
It has been rumored, but not demonstrated, that the term "Seeker" was used in some other early promotional materials. If so, then the term must have been handed down by Hasbro at some point. It hardly seems possible that the term could have been invented by a lowly copywriter at J.C. Penney who also just happened to decide to capitalize it. Still, though, if this catalog is the only primary source in which the term appears, it seems strange that it could have inspired the entire fandom to use the term. "Seeker" was in wide use among internet Transfans even in the early 1990s, when the fandom was just getting off its feet. Did somebody in the days of the Transformers email list have a memory of the word from when they saw this catalog at age 10 and start the trend? It is probably impossible to know at this point.
It is sometimes erroneously claimed that "Seeker" is derived from a line of dialog in the first episode of the G1 cartoon, in which these jets are referred to as "hunter-seekers". This line does not exist.
The term "hunter-seekers" is used in issue 17 of the US comic, "The Smelting Pool!". However, the Decepticons it is applied to are shown only in their flight modes, which look nothing like the "Seekers" that we are familiar with. In fact, the Conehead Seekers make their first appearance in this same issue, and their Cybertronian flight modes appear very similar to their Earth jet forms (if not completely identical), and completely different from the craft which are referred to as hunter-seekers. The only real connection between the hunter-seekers we were shown and the "Seekers" is that there's more than one of them and they fly.