Twitch
Definition 1 of 4
Pro Tip 1/2
Involuntary Reaction
Twitch as a verb often refers to unintentional body movements, not something one does on purpose. Whenever he hears a loud noise, his shoulders twitch.
Pro Tip 2/2
Sudden Movement
A twitch is usually a quick, sharp motion that happens suddenly, without a gradual buildup. The cat's ears would twitch at the slightest sound.