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 term.
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", although that is not used in all medical textbooks).
Hives usually occur as a result of an allergic reaction and the release of histamine. Although usually transitory, hives can be a medical emergency if the lips, eyes, or tongue are affected because considerable swelling can occur.