singlet

Concise Oxford English Dictionary © 2008 Oxford University Press:
singlet/ˈsɪŋɡlət/

▶noun

• 1 chiefly Brit. a vest or similar sleeveless garment.

• 2 Physics a single line in a spectrum, not part of a multiplet.

• 3 Physics a state or energy level with zero spin, giving a single value for a particular quantum number.

– origin C18 (orig. denoting a man's short jacket): from single (because the garment was unlined) + -et1, on the pattern of doublet.