Negative edge
An input quirk where releasing a button counts the same as pressing it, so a special move can fire on the release instead of the press.
The name is borrowed from electronics: the negative edge is the falling edge of a signal, the high→low transition — the moment the button comes up. Street Fighter II (1991) read its buttons so that both a press and a release could trigger a special, originally more an accident of the hardware than a design choice, later embraced.
The upside is that you can hold the attack button down through the motion input and release it on the final direction, smoothing your timing. The downside is just as plain: accidental specials whenever you happen to lift a finger. Modern engines therefore make it a toggle you can switch off.