The Collaborative International Dictionary of English v.0.48:

lactose intolerance \lac"tose` in*tol"er*ance\ (l[a^]k"t[=o]s`
   [i^]n*t[o^]l"[~e]r*ans), n.
   A physiological condition in which lactose cannot be
   hydrolyzed in the intestine, leading to uncomfortable
   gaseousness, cramps, or diarrhea after eating a
   lactose-containing food such as milk or ice cream. The
   symptoms may be prevented by ingesting a preparation of
   beta-galactosidase before or together with dairy products.
   [PJC]

WordNet (r) 3.0 (2006):

lactose intolerance
    n 1: congenital disorder consisting of an inability to digest
         milk and milk products; absence or deficiency of lactase
         results in an inability to hydrolyze lactose [syn: lactose
         intolerance, lactase deficiency, milk intolerance]