The Collaborative International Dictionary of English v.0.48:

lookup \look"up\ n. (Computers)
   an operation that determines whether one or more of a set of
   items has a specified property; as, they wrote a program to
   do a table lookup.

   Syn: search.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

lookup
    n 1: an operation that determines whether one or more of a set
         of items has a specified property; "they wrote a program to
         do a table lookup" [syn: search, lookup]