A T-shirt (T shirt or tee) is a shirt which is pulled on over the head to cover most of a person’s torso. A T-shirt is usually buttonless and collarless, with a round neck and short sleeves. However, many people incorrectly use the term T-shirt to describe any short sleeved shirt or blouse; a polo shirt or other collared shirt is not a T-shirt. The sleeves of the T-shirt extend at least slightly over the shoulder but not completely over the elbow (in short-sleeve version). A shirt that is either longer or shorter than this ceases to be a T-shirt. T-shirts are typically made of cotton or polyester fibers (or a mix of the two), knitted together in a jersey stitch that gives a T-shirt its distinctive soft texture. T-shirts can be decorated with text and/or pictures, and are sometimes used to advertise (see human billboard).
text source

The T-shirt evolved from undergarments used in the 19th century, through cutting the one-piece “union suit” underwear into separate top and bottom garments, with the top long enough to tuck under the waistband of the bottoms. T-shirts, with and without buttons, were adopted by miners and stevedores during the late 1800s as a convenient covering for hot environments.

T-shirts, as a slip on garment without buttons, originally became popular in the United States when they were issued by the U.S. Navy during or following the Spanish American War. These were a crew-necked, short-sleeved, white cotton undershirt to be worn under a uniform. It became common for sailors and Marines in work parties, the early submarines, and tropical climates to remove their uniform “jacket”, wearing (and soiling) only the undershirt. It is possible that the Navy uniform boards first discovered the T-shirt by watching dock crews.

Named the T-shirt due to the shape of the garment’s outline, they soon became popular as a bottom layer of clothing for workers in various industries, including agriculture. The T-shirt was easily fitted, easily cleaned, and inexpensive, and for this reason, it became the shirt of choice for young boys (perhaps more the choice of their mothers than of the boys themselves). Boys’ shirts were made in various colors and patterns.

By the time of the Great Depression, the T-shirt was often the default garment to be worn when doing farm or ranch chores, as well as other times when modesty called for a torso covering but conditions called for lightweight fabrics.

Following World War II it became common to see veterans wearing their uniform trousers with their T-shirts as casual clothing, and they became even more popular after Marlon Brando wore one in A Streetcar Named Desire, finally achieving status as fashionable, stand-alone, outer-wear garments.

text source