A singlet (also called a vest or sleeveless shirt) is a term used in the U.K. for a style of pullover shirt, that has shoulder straps and no sleeves. In the U.S., variations designed for women are known as tank tops or halter tops while the male version is called a sleeveless tee or a wife-beater (a derogatory term derived from films).
Singlets are usually made of cotton and have a round neck line or a v-neck but typically no collar. A plain white version is often worn as an undershirt or as working attire for performing manual labor. In schools students wear singlets in gym class (PE) and for sports activities in general. They are also worn as casual clothing in hot weather.
There are endless variations, many of which are related to sports teams, schools, and colleges which are identified by printed text and/or graphics.
Man with a singlet worn as an undershirt.