Hives are known by many names including "wheals", "weals", "nettle rash" which is a type of hive or wheal, and urticaria - which is the medical name.
Generally, hives are rounded or flat-topped elevated lesions caused by broad flares of swollen skin containing an excessive accumulation of fluid. (The medical term for "swollen with an excessive accumulation of fluid" is "edematous" - though this word is not used in all texts).
Hives usually occur as a result of an allergic reaction and the release of histamine. Although usually transitory,hives can constitute a medical emergency if the lips, eyes, or tongue are affected because considerable swelling can occur.