Young Blonde Girl Undresses Her Pink Sweater And Panties In The Studio

The term "sweater" is a catch-all for various types of 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; 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 t-shirt is not considered a sweater.