Periphery means the outer edge or boundary of something, the area farthest from the center. When you're standing in the middle of a playground, the swings and fence at the edges are on the periphery. When scientists study a cell under a microscope, they examine both its nucleus and its periphery, where the cell membrane meets the outside world.

The word often describes both physical boundaries and social ones. In a classroom discussion, students on the periphery might sit quietly while others dominate the conversation. During recess, a new student might stay on the periphery of a game, watching from the edges before joining in.

You'll also hear people talk about peripheral vision, which is what you see out of the corners of your eyes rather than straight ahead. Try staring at a single word on this page: you can still detect movement and shapes in your peripheral vision, even though they aren't in sharp focus.

Something peripheral is related to the periphery: it's on the edges, not central to the main action or purpose. When your teacher gives you peripheral details about a historical event, those facts are interesting but not essential to understanding the main story. The periphery is where things exist without being at the heart of what's happening.