Example of t-shirt undershirt, usually not worn as outwear
An undershirt is an article of underwear worn underneath a dress shirt intended to protect it from body sweat and odors. It can have short sleeves or be sleeveless. The term most commonly refers to upper-body wear worn by males.
It also makes dress shirts less transparent. It can also be worn during winter months as an extra layer of warmth.
Factory mass-produced undershirts became common in the West in the early 20th century, with innovations turning the union suit into two pieces, for upper and lower body.