The Collaborative International Dictionary of English v.0.48:

Erythema \Er`y*the"ma\, n. [NL., fr. Gr. ?, fr. ? to redden, fr.
   'eryqro`s red.] (Med.)
   A disease of the skin, in which a diffused inflammation forms
   rose-colored patches of variable size.
   [1913 Webster]

WordNet (r) 3.0 (2006):

erythema
    n 1: abnormal redness of the skin resulting from dilation of
         blood vessels (as in sunburn or inflammation)