Urticaria is a transient erythematous swelling of the skin, associated with itching, which usually resolves within 24 hours. It is caused by the degranulation of histamine-containing cells (mast cells) in the superficial dermis. Hives often appear as a raised, itchy rash. The medical name for hives is “urticaria”, also call it welts, wheals, or nettle rash.