Monocular means having one eye, using one eye, or designed for one eye.

A monocular device is like half of a pair of binoculars: a small telescope you hold up to one eye to see distant objects more clearly. Birdwatchers might carry a monocular in their pocket because it's lighter and more compact than binoculars, even though it provides only a single magnified view. Pirates in stories often wear eye patches and become monocular, seeing the world through just one eye.

Some animals have monocular vision, meaning their eyes point in different directions so each eye sees a separate view. A rabbit, for example, has monocular vision that lets it watch for danger on both sides at once, though it can't judge distances as well as humans can with our two forward-facing eyes working together.

The word contrasts with binocular (two eyes) and appears in science and medicine when discussing vision. An injury that affects only one eye can create monocular vision problems. In these contexts, monocular simply means “relating to or affecting one eye only.”