The Collaborative International Dictionary of English v.0.48:

Paronomasia \Par`o*no*ma"si*a\, n. [L., fr. Gr. ?, fr. ? to form
   a word by a slight change; para` beside + ? to name, fr.
   'o`noma a name.] (Rhet.)
   A play upon words; a figure by which the same word is used in
   different senses, or words similar in sound are set in
   opposition to each other, so as to give antithetical force to
   the sentence; punning. --Dryden.
   [1913 Webster] Paronomastic

WordNet (r) 3.0 (2006):

paronomasia
    n 1: a humorous play on words; "I do it for the pun of it"; "his
         constant punning irritated her" [syn: pun, punning,
         wordplay, paronomasia]