A device that switches or controls more than one electrical circuit such that they are turned on and off in a specific order with (sometimes) a delay between stages. Typically turns on devices first to last, but powers them off last to first. This keeps unwanted noises from going through the speakers at the power up/down by turning the mixer on first, then the processing gear, and the amplifiers last. (It's the reverse when powering down.) Even if a system doesn't have an automatic power sequencer, you should always turn the amplifiers on last and off first.