The Collaborative International Dictionary of English v.0.48:

Hemicrania \Hem`i*cra"ni*a\, n. [L.: cf. F. h['e]micr[^a]nie.
   See Cranium, and Megrim.] (Med.)
   A pain that affects only one side of the head.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hemicrania
    n 1: a severe recurring vascular headache; occurs more
         frequently in women than men [syn: migraine, megrim,
         sick headache, hemicrania]