The Collaborative International Dictionary of English v.0.48:

interlingua \in`ter*lin"gua\, n.
   A language used as an intermediate language in translating
   from a source language to a target language; -- used
   especially in machine translation by computers; as, some
   machine translation systems use Esperanto as an interlingua.
   [PJC]

WordNet (r) 3.0 (2006):

Interlingua
    n 1: an artificial language proposed for use as an auxiliary
         international language; based on words common to English
         and the Romance languages