A light typically collarless shirt or top made to be worn underneath other clothing, such as dress shirts. They're frequently plain and white.
Men may wear "wifebeater" tank top-style sleeveless undershirts, especially to accentuate a blue-collar look.
In terms of sportswear, the term "undershirt" usually refers to a taut shirt worn under most clothing (a baseball uniform, for example).
This tag should not be used for dress shirts.