undershirt

noun
  1. A piece of underwear worn under a shirt, typically a lightweight, sleeveless garment that provides an extra layer of warmth or absorbs sweat. noun
    He wore an undershirt beneath his dress shirt to stay warm.
    She bought a pack of cotton undershirts for her son to wear to school.
  2. A sleeveless or short-sleeved shirt worn as an outer garment, often in a casual setting. noun
    During the summer, he often wears an undershirt with shorts.
    The undershirt was comfortable enough to wear on its own during the hot day.