An undershirt is a lightweight shirt worn underneath your regular clothes, next to your skin. It's usually made of soft cotton or similar fabric and has short sleeves or no sleeves at all.

People wear undershirts for several practical reasons. They absorb sweat, which helps keep your outer shirt cleaner and fresher throughout the day. They add a layer of warmth in cold weather without the bulk of a heavy sweater. They also prevent scratchy fabrics from irritating your skin and can keep thin dress shirts from being see-through.

The most common style is the white cotton undershirt, which looks like a simple T-shirt. Another popular type is the tank top or sleeveless undershirt.

While undershirts are practical, they're usually meant to stay hidden under other clothes. One exception is sports: athletes often wear undershirts under their uniforms to stay comfortable during games.