The Collaborative International Dictionary of English v.0.48:

Catnip \Cat"nip`\, Catmint \Cat"mint`\, n. (Bot.)
   A well-know plant of the genus Nepeta (Nepeta Cataria),
   somewhat like mint, having a string scent, and sometimes used
   in medicine. It is so called because cats have a peculiar
   fondness for it.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Nepeta cataria
    n 1: hairy aromatic perennial herb having whorls of small white
         purple-spotted flowers in a terminal spike; used in the
         past as a domestic remedy; strongly attractive to cats
         [syn: catmint, catnip, Nepeta cataria]