Source : Webster's Revised Unabridged Dictionary (1913)

Index \In"dex\, v. t. [imp. & p. p. {Indexed}; p. pr. & vb. n.
   {Indexing}.]
   To provide with an index or table of references; to put into
   an index; as, to index a book, or its contents.

Source : WordNet®

indexing
     n : the act of classifying and providing an index in order to
         make items easier to retrieve