Peripheral describes something that exists at the edges or borders, rather than at the center. When something is peripheral to your main focus, it's off to the side, less important, or not quite central to what matters most.

Your peripheral vision is what you can see at the edges of your sight, without looking directly at it. If you're reading a book and notice your cat walking by, you spotted the movement with your peripheral vision. You didn't turn your head, but you still detected the motion happening at the boundary of what you could see.

In another sense, peripheral means less important or secondary. If a teacher asks about the main causes of the American Revolution and you start talking about what people ate for breakfast in 1776, that's peripheral information: interesting, but not central to the question. During a game of soccer, checking the scoreboard is important, but arguing about what color the grass is would be a peripheral concern.

In computing, a peripheral (noun) is a device you connect to a computer, like a mouse, keyboard, printer, or speakers. These devices exist at the edges of the computer system, adding functionality but not containing the computer's core processing power. The computer works without them, but they make it much more useful.