In electronics, trailing edge also describes the part of a wave (typically a square wave) where the voltage level is decreasing. Trailing edge can also be called falling edge. This is in contrast to the leading edge, where the voltage level is increasing. Trailing edge can also describe the last part of a signal pulse, where the voltage is returning to its normal, non-asserted level. The trailing edge of a pulse can have either decreasing or increasing voltage, depending on the assertion level of the pulse.
Let's look at a square wave:
+-----+ +-----+
| | | |
| | | | <---- Trailing edge
| | | |
| | |<-+ |
----+ +-----+ | +-
|
+---- Leading edge
Most microprocessors and other hardware devices use signal edges in order to activate functions such as interrupts, and while most are activated on leading edges, some also use trailing edges, and in some cases this choice can be programmed.