Website 1: A T-shirt (or tee shirt) is a shirt with short or long 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 (except that women often wear a bra under it).
Website 2: A short-sleeved, collarless undershirt
Website 3: A lightweight, usually knitted, pullover shirt, close-fitting and with a round neckline and short sleeves, worn as an undershirt or outer garment
Anyways, no website, no dictionary & no search engine could satisfy me with their worn out, old fashioned, incomplete definition. What they say is so bloody indigestable that it gives me severe constipation & let's not get into details of constipation n stuff.