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. For women, there may be some overlap with camisole. They may be worn in lieu of a bra by (typically younger) flat-chested girls with no need for supporting their breasts.
In terms of sportswear, the term "undershirt" usually refers to a shirt worn under most clothing (a baseball uniform, for example).
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