To twitch means to make a sudden, quick, involuntary movement. Your eye might twitch when you're tired, jerking slightly without you meaning to make it happen. A rabbit's nose twitches constantly, moving in quick little bursts as it sniffs the air. These movements are usually small and happen on their own, like when your leg twitches just as you're falling asleep.

The word can also describe a slight, nervous movement someone makes on purpose. Someone waiting for test results might twitch their fingers anxiously, or a baseball player might twitch his shoulders while standing in the batter's box, unable to stay completely still.

A twitch (as a noun) is the movement itself: “I felt a twitch in my shoulder.” Animals like horses twitch to shake off flies without having to move their whole bodies.

People sometimes call something “twitchy” when it's jumpy or overly sensitive, like a twitchy car alarm that goes off at the slightest touch, or a twitchy squirrel that darts away at any sudden sound.