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Medical Definition of Heat urticaria
1. A form of physical or non-allergic urticaria initiated by heat (e.g., hot baths, physical exercise, pyrexia, exposure to sun or to a warm room) or by excitement; the rather distinctive lesions consist of pruritic areas 1 to 2 mm in diameter surrounded by bright red macules. Synonym: heat urticaria. (05 Mar 2000)