The Collaborative International Dictionary of English v.0.48:

Pluralize \Plu"ral*ize\, v. t. [imp. & p. p. Pluralized; p.
   pr. & vb. n. Pluralizing.]
   1. To make plural by using the plural termination; to
      attribute plurality to; to express in the plural form.
      [1913 Webster]

   2. To multiply; to make manifold. [R.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pluralize \Plu"ral*ize\, v. i.
   1. To take a plural; to assume a plural form; as, a noun
      pluralizes. --Earle.
      [1913 Webster]

   2. (Eccl.) To hold more than one benefice at the same time.
      [Eng.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

pluralize
    v 1: mark with a grammatical morpheme that indicates plural;
         "How do speakers pluralize nouns in Japanese?" [syn:
         pluralize, pluralise]