Newly added terms are in bold and should be dated.
Aerospace Fighter-A close second to the battlemech in terms of battlefield effectiveness, aerospace fighters are similar to standard fighters with the exception that they can effectively operate in space and in planetary atmospheres. Like battlemechs, aerospace fighters have a model name and often have a personal name granted by their pilot.
Assault-The heaviest weight class (see Weight Classes). Also a modifier for several weapons systems.
Battlemech-The central war machine of the setting. Tanks on legs, they are kings of the battlefield, largely more effective in most cases than infantry, vehicles, and aerospace fighters. Battlemechs come in a series of identifiable names. Most 'mechs of the same model have different variants, but hold a similar battlefield role. That said, given the common practice of field-modifying 'mechs and the proliferation of factory variants, this isn't always the case. In addition, many 'mech pilots, or mechwarriors, tend to give a name to their personal 'mech.
Binary-A Clan formation of two stars.
Cluster-A Clan formation of four to five binaries or trinaries.
Company-A company usually consists of three lances.
Coolant-Highly toxic fluid used to cool the massive heat generated by various machinery, in this context, Battlemechs. Fumes are harmless, but distinct.
Dropships-Dropships carry goods and supplies from Interstellar Jumpships to planets and facilities in a solar system and vice versa.
Exterminator - A technological marvel before the decline of technology, it's a heavy 'mech designed for light engagement and headhunting missions behind enemy lines. The most notable attribute of the Exterminator is the extensive stealth and detection avoidance systems. The use of these systems is considered dishonorable to Clan sensibilities, and they are not utilized by the Clan warriors unfortunate enough to be assigned these machines.
Fiat-Confirmation that everything you're reading is written by a guy who's far more credulous about this story than you are. (19-Feb)
Galaxy-A Clan formation of three to five clusters.
Ghost Bears-A Clan.
Great Cost-Offhand remarks made to pad out heroic wins. Usually measured in peripheral characters killed and greviousness of predictions about the unit's future combat ability. (19-Feb)
Heavy-The second-heaviest weight class (see Weight Classes). Also a modifier for several weapons systems.
Heroic Wins-See Fiat. (19-Feb)
Holovid-Space TV. Often viewed as a pulp format and more often viewed by Battletech novelists as a venue for self-depricating, self-aware humor (or attempts thereof).
Jade Falcons-A Clan.
Jumping-Some battlemechs possess the ability to superheat air and expell it in such a way that they can generate thrust. Enough thrust to propel a 20, 50, for even 100 ton war machine into the air and over (or into) all manner of obstacles and enemy units.
Jumpship-Interstellar craft capable of transporting goods between stars, but not within star systems.
Lance-A lance usually consists of four battlemechs or vehicles.
Light-The lightest weight class (see Weight Classes). Also a modifier for several weapons systems (Light Autocannon, Light PPC, etc.).
Mad Cat-See Timberwolf
Mechwarrior-A common term for someone who pilots a battlemech. Also, the lowest rank of Clan mechwarriors.
Medium-The second-lowest weight class (see Weight Classes). Also a modifier for several weapons systems.
Not-Named Clan-Initially named, it's actions earned it the universal scorn of its peers and had its name striken from the records (Mongooses. Admittedly, it's a good team name.)
Nova Cats-A Clan that turned against the Smoke Jaguars and started working with the Inner Sphere.
Omega Galaxy-A Clan Ghost Bear unit not held to usual Clan traditions of warfare. They often use tactics akin to those of the Inner Sphere, and often more effectively.
Omnimech-A type of battlemech (usually Clan), which uses modular weapons pods for easy mission specialization and dramatically reduced repair times.
Omni-Short for 'omnimech'
Protomech-Tiny battlemechs with all of the drawbacks of Battlemechs, plus a few extra for being smaller. They utilize mental commands of tiny pilots curled up inside of their tiny cockpits. Scientists being as indisposed towards cruelty as they are, they are designed so pilots go mad long before the claustrophobia sets in. (19-Feb)
Smoke Jaguars-A Clan, one of the two that invaded Luthien, only to be repulsed by the inferior forces present (see 'Great Cost'). (19-Feb)
Star Commander-A Clan rank suitable for someone who commands a star.
Star League-The Star League was an organizational body that exemplifies the pre-decline civilization of The Inner Sphere. When it was undone by treachery and (wait for it) warfare, its pacifying influence was lost and the Succession Wars began, destroying the civilization it had served to create. The Star League is analgous to Camelot in the minds of many in the Battletech setting, both in The Clans and the Inner Sphere. In the minds of many players, it's the fascist oppressor whose ruthless government merely kept one ethnic group from cleansing another.
Star-A Clan battlemech formation that consists of five battlemechs (or omnimechs).
Tauman-A Clan's collective military forces. Distinct from hippes telling you about the best snipers in Warhammer 40k. (19-Feb)
Timberwolf - Also known as a Mad Cat, the Timberwolf is the cutting edge of Clan technology. Tough, potent, and quick it's an iconic 'mech of the setting, and with good damned reason.
Trials-A more formal way to kill one another than warfare. Trials come in a handful of types, including Greviance(I hate you), Posession(I'm taking your shit), Absorption(I 'm taking your shit and you'll like it), and Annihilation(I'm killing you and no one will miss you when you're gone). (19-Feb)
Trinary-A Clan formation of three stars.
Weight Classes-A rough categorization of units by their weight. Heavier units are usually slower, better armed, and better armored, while lighter units are faster, less potent, and easier to destroy (if hit). Light, Medium, Heavy, and Assault are the standard weight classes, with Superheavy being a speculative addition to the higher end.
Zellbrigen-A sort of formal challenge within The Clans where two guys specifically shoot at each other and no one else. This varies from Clan to Clan, as some will modify this to include more than two guys, sometimes shooting at others as well. (19-Feb)