The Collaborative International Dictionary of English v.0.48:

lemmatize \lem"ma*tize\ (l[e^]m"m[.a]*t[imac]z`), v. t.
   To convert into a lemma[2]; to normalize the form of (a word)
   to that form used as the headword in a dictionary, glossary,
   or index; as, "ride" is the lemmatized form for "riding" and
   "ridden".
   [PJC]