A T-shirt is a shirt with short sleeves, a round neck, put on over the head, without pockets. It was originally used as undershirt. This is still done, but it is also often worn as the only clothing on the upper part of the body
A T-shirt (or tee shirt) is a shirt, usually buttonless, collarless, and pocketless, with a round neck and short sleeves, that is pulled on over the head and covers most of a person's torso. The sleeves of the T-shirt extend at least slightly over the shoulder but not completely over the elbow. A shirt that is either longer or shorter than this ceases to be a T-shirt, at least in the classic sense.