The Collaborative International Dictionary of English v.0.48:

Singularize \Sin"gu*lar*ize\ (s[i^][ng]"g[-u]*l[~e]r*[imac]z),
   v. t.
   To make singular or single; to distinguish. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

singularize
    v 1: distinguish as singular [syn: singularize, singularise]