Special moves (aka specials) comprise one of the categories of moves. They generally typify a character’s abilities. The Japanese name is hissatsu waza, meaning ‘deadly move’; the term tokushu waza (which literally means ‘special move’) actually refers to command moves.
Every game has its own definition, but the vast majority require a sequence of stick motions, yielding higher-ranking moves than normal moves.
3D games in particular don’t have much of a distinction between normal moves and special moves; character-specific techniques fall under their own category (see “characteristic” moves).
In general, they have more offensive output than normal moves, but in return leave one more open. In most 2D fighting games, they do chip damage.
Super special moves and such are also included as part of this class; the definition is a little unclear, because different games often have their own ways of handling them.
In some games, there are strengthened versions of special moves: greater than special moves, lesser than super special moves. They often take a portion of the super meter, but less than a super special move would.