dilate

An eye dilates when it gets bigger or changes shape so that more light comes in. You know how sometimes your eyes feel wide and relaxed, like you're staring at a beautiful sunset?

When we talk about dilating something, we usually mean making it wider or increasing its size. Doctors might use this word to describe what happens to the pupils of our eyes when they get bigger to let more light in. But if someone is feeling very emotional, their heart might dilate, meaning it beats faster and stronger because it's working harder. The opposite of dilating is constricting, where something gets smaller or tighter.