The Collaborative International Dictionary of English v.0.48:

allergy \allergy\ n.
   1. hypersensitivity to a particular substance. "an allergy to
      bee venom so severe that a second sting may be fatal"
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

allergy
    n 1: hypersensitivity reaction to a particular allergen;
         symptoms can vary greatly in intensity [syn: allergy,
         allergic reaction]