The Collaborative International Dictionary of English v.0.48:

Headache \Head"ache`\ (h[e^]d"[=a]k`), n.
   Pain in the head; cephalalgia. "Headaches and shivering
   fits." --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

headache
    n 1: something or someone that causes anxiety; a source of
         unhappiness; "New York traffic is a constant concern";
         "it's a major worry" [syn: concern, worry, headache,
         vexation]
    2: pain in the head caused by dilation of cerebral arteries or
       muscle contractions or a reaction to drugs [syn: headache,
       head ache, cephalalgia]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "headache":
   MS, ache, aching, ado, aggravation, amyotrophic lateral sclerosis,
   angina, annoyance, anxiety, backache, bad news, bane, bedevilment,
   bellyache, besetment, bore, bother, botheration, bothersomeness,
   brain disease, buttonholer, can of worms, cephalalgia,
   cerebral palsy, chorea, colic, collywobbles, crashing bore,
   devilment, difficulty, disadvantage, dogging, downer, drag, drip,
   dryasdust, dusty, earache, emotional disorder, epilepsy, evil,
   exasperation, falling sickness, flat tire, fret, frightful bore,
   glossopharyngeal neuralgia, gnawing, great ado, gripe, gripes,
   gut-ache, harassment, harrying, heartburn, hemicrania,
   herpes zoster, hounding, humdrum, inconvenience, ischialgia,
   matter, megrim, migraine, molestation, multiple sclerosis,
   nervous disorder, neuralgia, neuritis, neuropathy, nuisance,
   odontalgia, organic psychosis, otalgia, palsy, peck of troubles,
   persecution, pest, pill, polyneuritis, pressure neuropathy,
   priapism, problem, proser, pyrosis, radiculitis, sciatic neuritis,
   sciatica, sea of troubles, shaking palsy, shingles, sick headache,
   spastic paralysis, splitting headache, stomachache, the jerks,
   throbbing pain, tic douloureux, toothache, toxic psychosis, trial,
   trouble, twaddler, vexation, vexatiousness, wet blanket, worriment,
   worry