the GNU version of the Collaborative International Dictionary of English
n. A pain that affects only one side of the head.
The Century Dictionary and Cyclopedia
n. In pathology, headache on one side of the head; especially, megrim when confined to one side; also, megrim in any form.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. a severe recurring vascular headache; occurs more frequently in women than men
Word Usage
"It affects about ten per cent of the population and comes from the Greek word hemicrania, meaning half the head; this refers to the often one-sided excruciating pain."