A shirt is a cloth garment for the upper body (from the neck to the waist).
Originally an undergarment worn exclusively by men, it has become, in American English, a catch-all term for a broad variety of upper-body garments and undergarments. In British English, a shirt is more specifically a garment with a collar, sleeves with cuffs, and a full vertical opening with buttons or snaps (North Americans would call that a "dress shirt", a specific type of "collared shirt"). A shirt can also be worn with a necktie under the shirt collar.