A boss, in fighting games, is a strong enemy that one must face at specific points in a human vs. A.I. game.
A mid boss (also “miniboss”) is a boss that shows up before the last boss, and usually between cutscenes (and often, contrary to the name, is the fight right before the last boss). Many games do not have one. Other times, mid bosses can be a weak version of the last boss, right before they take a “true form”.
Oftentimes, mid bosses may only appear (or replace the normal mid boss) if certain conditions are met. Other times, they may not really be bosses; they are normal or secret characters that show up challenge you, and beating them will often “unlock” them (so you can use them).
Mid bosses tend not to have all the crazy balance-breaking abilities that the last bosses do, and lots of them are playable characters (or characters that are still considered fair game to use once unlocked). Some of them get downgraded to normal characters in sequels or revisions (such as the three mid-bosses and the last boss in SFII).
In story-type arcade modes, there is sometimes a mid-boss match against a perfectly normal character (usually due to a rivalry of some sort). When this happens, there tends to be a short conversation between the two, and sometimes the game system even pretends that someone is challenging you like another human player would.
Also known as the final boss, or just “the” boss. The last boss is found in most fighting games as an extremely cheap character; if you beat them, you will see the ending and credits (usually). Depending on the game, you may also come across a secret boss if you meet certain conditions, or there may be a “true” boss, separate from the normal “last” boss.
Some games give you a different last boss depending on which character you use (e.g., Street Fighter Alpha and Alpha 2, and Sengoku Basara X). In this case, the last boss may even be anoter normal playable character, although the AI may be ramped up and artificial advantages may be given to that character (such as increased damage per hit).
The majority of bosses, and particularly last bosses, have all sorts of incredibly high abilities, and particularly defensive ability; beginners have pretty much no shot at beating them. When one gets into the habit of figuring out their patterns and exploiting their few weak spots, however, they stop seeming so bad.
Developers of fighting games always try to find new ways to distinguish their bosses. Particular boss moves such as Geese Howard’s Atemi Nage (the first trap move), Gōki’s Shungokusatsu, Orochi’s Marokare (floods the whole screen with light), and Gill’s Resurrection (thought you beat him? think again!) become very iconic. Sometimes, some odd property of the boss is the creative hook: Apocalypse (from a couple of the Marvel games), for example, is in fact much larger than the screen.
Some games have the last boss as a playable character, even in the arcade version; however, the AI usually gets to use that character with capabilities added on that the player’s version does not have (e.g., the player’s version will usually have a subset of the boss version’s moves, the moves will be weaker, and the player’s move commands require charging, etc.). Regardless, these cases still usually rank the character in a high tier.
Home ports of arcade fighting games will often let you play as boss characters. Some games do not include this feature, however.
Opinion varies widely, but many consider General from Kaiser Knuckle to be the hardest. The secret boss Parace L’sia from Arcana Heart 2 comes in a close second.
Other brutal challenges include Shin Gōki from SFA2, Magneto from X-Men: Children of the Atom, Igniz from KOF 2001, and Gönitz from KOF ’96.
In the hands of a player, Orochi from KOF ’97, Onslaught from Marvel vs. Capcom, and Omega Rugal from KOF ’98 are terrifying.
Lots of bosses are popular compared to normal characters, often because they have a lot to their personality and looks. In particular, some of SNK’s most outrageous villains are popular despite originally being a one-show, probably on account of their personality, like Geese, Rugal, Orochi, and Gönitz (well, in Japan, anyhow).
Some bosses do not have a crouching graphic (even if they can crouch system-wise). This happens when developers cut corners and decide not to have crouching factor into the boss’ pattern.
This gets very confusing, and sometimes a console port will have graphics added so that the boss can crouch.
A sort of boss that shows up instead of the usual last boss if you meet certain conditions (e.g., Mukai instead of Adelheid in KOF 2003).
A sort of boss that shows up after the usual last boss if you meet certain conditions (e.g., Kain after Grant in Mark of the Wolves).
Since this is usually done in the spirit of “omake”, they may actually be weaker than the “last” boss.