The Collaborative International Dictionary of English v.0.48:

Megrim \Me"grim\, n. [OE. migrim, migrene, F. migraine, LL.
   hemigrania, L. hemicrania, hemicranium, Gr. ?; ?- half + ?
   skull. See Hemi- and Cranium, and cf. Hemicrania,
   Migraine.]
   1. A kind of sick or nervous headache, usually periodical and
      confined to one side of the head; now more commonly called
      migraine headache or migraine.
      [1913 Webster +PJC]

   2. A fancy; a whim; a freak; a humor; esp., in the plural,
      lowness of spirits.
      [1913 Webster]

            These are his megrims, firks, and melancholies.
                                                  --Ford.
      [1913 Webster]

   3. pl. (Far.) A sudden vertigo in a horse, succeeded
      sometimes by unconsciousness, produced by an excess of
      blood in the brain; a mild form of apoplexy. --Youatt.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

migraine \mi*graine"\, n. [F. migraine, LL. hemigrania, L.
   hemicrania, hemicranium, Gr. "hmikrani`a; "hmi- half +
   krani`on skull. See Hemi- and Cranium, and cf.
   Hemicrania, Migraine.]
   1. A kind of intense sick or nervous headache, usually
      periodical and confined to one side of the head; called
      also migraine headache. Same as megrim. --
      Mi*grain"ous, a.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "migraine":
   MS, ache, aching, amyotrophic lateral sclerosis, angina, backache,
   bellyache, brain disease, cephalalgia, cerebral palsy, chorea,
   colic, collywobbles, earache, emotional disorder, epilepsy,
   falling sickness, fret, glossopharyngeal neuralgia, gnawing, gripe,
   gripes, gut-ache, headache, heartburn, hemicrania, herpes zoster,
   ischialgia, megrim, multiple sclerosis, nervous disorder,
   neuralgia, neuritis, neuropathy, odontalgia, organic psychosis,
   otalgia, palsy, polyneuritis, pressure neuropathy, priapism,
   pyrosis, radiculitis, sciatic neuritis, sciatica, shaking palsy,
   shingles, sick headache, spastic paralysis, splitting headache,
   stomachache, the jerks, throbbing pain, tic douloureux, toothache,
   toxic psychosis