The Collaborative International Dictionary of English v.0.48:

Enallage \E*nal"la*ge\, n. [L., fr. Gr. ? an exchange, fr. ? to
   exchange; ? in + ? to change.] (Gram.)
   A substitution, as of one part of speech for another, of one
   gender, number, case, person, tense, mode, or voice, of the
   same word, for another.
   [1913 Webster]

WordNet (r) 3.0 (2006):

enallage
    n 1: a substitution of part of speech or gender or number or
         tense etc. (e.g., editorial `we' for `I')