EX refers to the gameplay mechanic from the later Street Fighter games, and the spinoff Street Fighter EX series.
The most common usage of EX as a gameplay mechanic introduced in the Street Fighter 3 series (originally introduced as ES attacks in [Dark Stalkers]). Players can spend a certain amount of super meter when performing a special move to give it bonus damage or properties. To activate an EX move, commonly a player has to press two buttons instead of one when activating the special move, provided he/she has enough super meter. This mechanic has been carried over into Street Fighter IV and Street Fighter V. Informally used to refer to powered up versions of special attacks in many games.
EX may also refer to a series of the Street Fighter Series featuring polygonal graphics and 3D fighting game mechanics.