A shutter is a movable cover that opens and closes. Most often, you'll see shutters on windows: wooden or metal panels on hinges that swing closed to block light, keep out storms, or add privacy. Many old houses have shutters that actually work, though modern ones are often just decorative.

Cameras also have shutters, but these work very differently. A camera's shutter is a mechanism inside that opens briefly to let light hit the film or sensor, then snaps shut again. When you press the button to take a picture, you're triggering the shutter. The faster the shutter speed, the less time light has to enter, which helps freeze fast action like a hummingbird's wings or a soccer ball in mid-kick.

You might also hear someone say a business shuttered its doors, meaning it closed permanently. A shop that shutters is one that shuts down for good.