noun

1. Brit. a man's undershirt, esp. a sleeveless one
2. Austral., N.Z. a short-sleeved or sleeveless knit top, as a T-shirt or tank top
3. any single thing that usually exists in combination; specif., an atom that is freed from combination by being raised to a higher energy level, as an atom of oxygen in a cell excited by a laser or sunlight, that becomes destructive

Etymology: < single, modeled on doublet