Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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