nebula

Concise Oxford English Dictionary © 2008 Oxford University Press:
nebula /ˈnɛbjʊlə/

▶noun (pl. nebulae /-liː/ or nebulas)

• 1 Astronomy a visible cloud of gas or dust in outer space.

  ■ old-fashioned term for galaxy.

• 2 Medicine a clouded spot on the cornea causing defective vision.

– derivatives
nebular adjective.

– origin C17: from L., lit. ‘mist’.