In keyboard terms, a power button or power key (depending on the design) is a button or key on a keyboard used to switch on the computer. In many cases, regardless of the keycap form factor, it uses the same switch as the rest of the keyboard.
The power button or key is a soft power control that is wired up as a regular key on the keyboard matrix: the keyboard interface on the motherboard (e.g. ADB) must be left permanently running. As a normal key, it can also be detected by the operating system: many versions of Mac OS display a shutdown dialog in response to this key being pressed.
Macintosh keyboards featured a single power button, labelled with a hollow left-facing triangle. Keyboards designed for Windows sometimes feature a bank of keys for controlling power and sleep states.
Power button on a Filco FKB-97S keyboard