1. A soft, silvery, highly reactive element that is an alkali metal and is used as a heat transfer medium, in thermonuclear weapons, and in batteries, lubricants, various alloys, ceramics, and optical glass. Atomic number 3; atomic weight 6.941; melting point 180.5°C; boiling point 1,342°C; specific gravity 0.534; valence 1. See Periodic Table.

2. Any of several salts of lithium, especially lithium carbonate.