Lewis acid is a chemical species that reacts with a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that donates a pair of electrons to a Lewis acid to form a Lewis adduct. For example, OH− and NH3 are Lewis bases, because they can donate a lone pair of electrons.
Links
[1] http://hootsgo.com/?q=node/6470
[2] http://en.wikipedia.org/wiki/Lewis_acids_and_bases