Source : Webster's Revised Unabridged Dictionary (1913)

Headache \Head"ache`\, n.
   Pain in the head; cephalalgia. ``Headaches and shivering
   fits.'' --Macaulay.

Source : WordNet®

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}, {vexation}]
     2: pain in the head caused by dilation of cerebral arteries or
        muscle contractions or a reaction to drugs [syn: {head
        ache}, {cephalalgia}]