What does "nitrous" mean?

Nitrous: (Chem.) Of, pertaining to, or designating, any one of those compounds in which nitrogen has a relatively lower valence as contrasted with nitric compounds. Nitrous acid (Chem.), a hypothetical acid of nitrogen HNO2, not known in the free state, but forming a well known series of salts, viz., the nitrites. -- Nitrous oxide. See Laughing gas.