noun, plural pe·riph·er·ies. the external boundary of any surface or area.

What does it mean to periphery?

Periphery means outside the boundary of something. If you're on the periphery of a group, you're close to it but not part of it. A band on the periphery of a particular scene might have opened for another band a few times, but it's never headlined or gotten their big break.

Is it periphery or peripheral?

In general, peripheral is something that is on the side, just outside of the main focus or issue. Yes, I'm ignoring the computer device aspect in this. The periphery is the edge, a “line” or boundary of something.