Unseen refers to certain imagery, namely the visual appearance of a number of iconic BattleMechs, that is not depicted in official publications anymore since 1994 for legal reasons.
To wit, only their visual appearance may not be officially used anymore; the units as such (particularly their names and game stats) remain part of the BattleTech canon and are even frequently mentioned in novels and fiction. Neither the images nor the units they represent were retconned out.
The term is alternatively used to summarily refer to the real-world imagery as such, to the particular in-universe designs (and subvariants) in question, or as an adjective.
The list of Unseen has repeatedly changed over time, with some individual designs put on and off the list several times.
- Main article: Unseen lawsuits
When FASA created the BattleTech boardgame in 1984, the designers licensed the rights to use pre-existing Mecha from other IPs, namely the Japanese anime series' The Super Dimension Fortress Macross, Fang of the Sun Dougram and Crusher Joe as visual representation of several 'Mechs and other units within the BattleTech universe (under new names and with BattleTech game stats devised entirely by FASA). However, during a series of lawsuits in 1994 FASA found out that TCI, from whom FASA's rights to the images derived, might not have been in a legal position to provide these rights to FASA in the first place. The lawsuits were settled out of court.
As part of the settlement agreement, FASA lost the right to use the images henceforth. As this affected many core designs which have been described as the bedrock of the BattleTech universe at the time, this was a serious blow to FASA. All existing products featuring pictures of the Unseen, be it on the cover or in the rulebooks—effectively most if not all BattleTech publications—had to be discontinued, and while numerous other designs remained that could still be depicted, this massively affected the BattleTech line.
Beyond the specific artwork covered in the Unseen lawsuits, FASA decided to treat all designs as Unseen that had been developed out-of-house, such as the designs made for FASA by Victor Musical Industries. As a result, there are now several kinds of unseen images:
- images to which FASA found might never have legally acquired the rights to, and which they are not allowed to use or show;
- images to which FASA may not initially have acquired the rights, but which they are allowed to use or show after a later clarification agreement with the IP owners (but which, being out-of-house artwork, are still not used on general principle);
- images which were never contested and to which FASA did obtain the rights, but which, being out-of-house artwork, are not used on general principle;
- images that were never actually declared unseen, but should arguably be considered unseen since they originate from the same sources and legal background as the original unseen.
In 2003, Technical Readout: Project Phoenix was published that postulated upgrades and newer versions for most of the Unseen units from the in-game year 3067 forthwith. Dubbed "Reseen", these evolutions were sufficiently visually different from their parent designs that they could be depicted again.
To wit, the Reseen did not replace the Unseen or retcon them out. They are new, additional designs with new art, meant to gradually replace the Unseen as the timeline progresses.
The term "reseen" is also applied to designs that were unseen at one point, but were taken off the list again.
 25 Years of Art and Fiction
On 24 June, 2009, it was announced that Catalyst Game Labs would resume use of the original "unseen" artwork for these designs. The announcement did not mention whether or not Catalyst Game Labs had secured the permissions needed. 
Shortly after the return of the "unseen" BattleMechs was announced, the BattleTech: 25 Years of Art & Fiction PDF was pulled from circulation. Managing editor Randall Bills made a statement on Catalyst Game Labs' website on August 10th, 2009 explaining the move. Twelve Unseen images, those originating from Macross, were again pulled from circulation and will remain unseen, due to their exclusive distribution rights (within the United States) belonging to another company (Harmony Gold). Whether or not these images will be licensed or purchased in the future remains to be seen. The tone of the article was doubtful.
 List of Unseen designs
The Unseen designs, and their origins, are:
 From Crusher Joe
- Samurai AerosSpace Fighter
- Mix of Siren and Harpy
- Sparrowhawk AeroSpace Fighter
- Galleon Tank
- Galleon Tank
 From Macross
- VF-1 Valkyrie
This 'Mech from the Super Dimension Fortress Macross series normally transformed into the three combat modes recognized in BattleTech by Land Air 'Mechs. Within the BattleTech game this one vehicle was split into distinct non-transformable 'Mechs models and LAM models. The LAMs, true to their artistic origin, could transform into aerospace fighter, hybrid, and 'Mech mode.
- Stinger VF-1A
- Stinger LAM - LAM model of above, illustrated in LAM or Hybrid mode
- Wasp VF-1S
- Wasp LAM - LAM model of above illustrated in LAM or Hybrid mode
- Super Wasp
- Valkyrie VF-1S
- Phoenix Hawk - VF-1S with FAST Packs
- Phoenix Hawk IIC - VF-1S :Reseen under a new agreement
- Phoenix Hawk LAM - LAM model of above illustrated in LAM or Hybrid mode
- Crusader - Armored VF-1A, 
- Falcon :Reseen as of Record Sheets: Operation Klondike (From BattleDroids)
- Firebee (From BattleDroids)
- Hornet (From BattleDroids)
- Zentraedi "Regult" Tactical Battle Pod
- HWR-00-Mk. II Monster Heavy Weight Destroid
- MBR-07-Mk II Spartan Main Battle Destroid
- MBR-04-Mk VI Tomahawk Main Battle Destroid
- MBR-04-Mk X Defender Anti-Aircraft Defense Destroid
- SDR-04-Mk XII Phalanx Space Defense Destroid
- Zentraedi "Glaug" Officer's Battle Pod
 From Dougram
- Shadow Hawk :Reseen as of Record Sheets: Operation Klondike
- Shadow Hawk IIC :Reseen under a new agreement
- H8 Roundfacer
- F35C Blizzard Gunner
- T10B Blockhead
- F4X Ironfoot
- F44A Crab Gunner
- HT128 Bigfoot
- Battlemaster IIC :Reseen under a new agreement
 From Victor Musical Industries
These have been temporary Unseen, until becoming Reseen.
- Conjurer (Hellhound)
- Glass Spider (Galahad)
- Howler (Baboon)
- Bane (Kraken)
- Vapor Eagle (Goshawk)
- Black Python (Viper)
- Stone Rhino (Behemoth)
- Incubus (Vixen)
- Grand Crusader
- ↑ Catalyst Game Labs website, "Catalyst Game Labs Brings Back “Unseen”"
- ↑ Catalyst Game Labs website, "Sometimes Things Just Don’t Go As We Want Them To"
- ↑ In some artwork such as in The Fourth Succession War Scenarios Volume One, the Crusader is depicted with a modified VF-1J head.