A small-signal switching diode is a kind of semiconductor diode specially designed and manufactured for switching "on" and "off" on circuits. It takes less time to change from on to off or vice versa than ordinary diodes.
The time from off (high resistance) to on (low resistance) of a small-signal switching diode is called turn-on time. The time from on to off is called reverse recovery time. The sum of the two is called switching time. Generally, the reverse recovery time is longer than the turn-on time, so only the reverse recovery time is given on the operating parameters of the switching diode.