sweater or pullover

A sweater, jumper, pullover, jersey or guernsey is a knit garment intended to cover the torso and arms. It is often worn over a shirt, blouse, T-shirt, or other top, but may also be worn alone as a top. Sweaters tend to be, and in earlier times always were, made from wool; however, they can be made of cotton, synthetic fibers, or some combination thereof. Sweaters are maintained by washing, and the use of a lint roller.

The term "sweater" is a catch-all for various types of knit garments. A fundamental division is between a cardigan (which opens in front) and a pullover (which does not). If the sweater has no sleeves, it is generally called a sweater vest or tank top, or, in the UK, very commonly a "sleeveless pullover" which will very usually have a V neck and can be worn semi-formally; in UK parlance such a sleeveless garment would only be called a "tank top" if it had a square or scooped neck and a generally juvenile appearance.; a "vest" in British English refers to the same item as "undershirt" in American English.
A garment of this type was originally named a sweater either because it was designed to absorb sweat,or the garment was designed to promote a sweat during exercise. It has now become a household name, and other styles of garment (as named above) have kept the original term. A thermal shirt is considered a sweater.