A keyhole is the small opening in a lock where you insert a key. The shape of the keyhole matches the shape of the key's blade, so only the right key will fit and turn the lock mechanism inside. Traditional keyholes are often shaped like a circle with a narrow slot below it, though modern locks come in many different shapes.

Before electric lights were common, keyholes served another purpose: people would peek through them to see what was happening on the other side of a locked door. This is why in old cartoons and movies, you'll often see a character crouching down to spy through a keyhole, with a bright circle of light showing what they can see. In reality, keyholes offer only a tiny, limited view.

The phrase keyhole view or keyhole perspective now describes a very narrow, restricted way of seeing something. If you only hear one side of an argument between friends, you have a keyhole view of what really happened. Scientists might talk about having only a keyhole glimpse into how something works when they can observe just a small part of a larger system. The word reminds us that sometimes what we can see is much smaller than the whole picture.