The Collaborative International Dictionary of English v.0.48:

Split infinitive \Split infinitive\ (Gram.)
   A simple infinitive with to, having a modifier between the
   verb and the to; as in, to largely decrease. Called also
   cleft infinitive.

   Note: The use of the split infinitive is commonly considered
         to be ungrammatical, but by most grammarians it is
         considered acceptible.
         [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

split infinitive
    n 1: an infinitive with an adverb between `to' and the verb
         (e.g., `to boldly go')