A dash is an action that allows one to run forward instead of walking. It is usually executed by tapping forward twice on the stick. This run can be continued by continuing to hold the stick in its direction.
In many cases, one may put out an attack immediately during the period of running, and some games exclusive dash attacks in them.
The original dash was implemented in the first Art of Fighting.
In Capcom vs. SNK 2, with influence from the Darkstalkers and Marvel series, a frontstep is called “dash”, and a dash is called “run”. Morrigan can do this “run” in midair (unique out of the whole cast).
There are some games (such as NEO·GEO Battle Coliseum, Pocket Fighter, and Rival Schools) in which the difference between a frontstep and a dash is whether or not the stick is held in position.
In other games (such as Mark of the Wolves), whether or not you perform a dash or a frontstep depends on the character you use.