The Collaborative International Dictionary of English v.0.48:

Nominalize \Nom"i*nal*ize\, v. t. (Grammar, Linguistics)
   To convert into a noun; as, a nominalized sentence; a
   nominalized adjective or verb.
   [1913 Webster]

   Note: In a sentence of the form:
         "That Kennedy would have been a great president if he
         had not been assassinated is one of the enduring myths
         of the Camelot conceit."
         the sentence "Kennedy would have been a great president
         if he had not been assassinated" has been nominalized
         by the introductory "that".
         [PJC]