Wah: A wah is a variable equalization boost pedal. It was invented by trying to redesign a midrange boost feature on an amplifier. They wanted to change the on/off switch to a pot, which would have been cheaper to produce.
When they rotated the pot throughout it’s range, it made a very vocal-like “wah-wah” sound. This is paramount to the wah effect, as the classic sound is created by the change of tone from sweeping this pot, as opposed to just turning the effect on. This would be very hard for a player to adjust while playing however, so they housed it in an expression pedal for foot control.
Simpler Definition: Wah Wah Sound
Effect Pedal Examples:
Fifty Favorite Effect Pedals for Sale: