Urticaria is known by many names including hives which is the most common name, "wheals", "weals", and "nettle rash" which is a type of hive or wheal.
Generally, urticaria are rounded or flat-topped elevated lesions caused by flares of swollen skin containing an 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.
They usually occur as a result of an allergic reaction and the release of histamine. Although usually transitory, urticaria can constitute a medical emergency if the lips, eyes, or tongue are affected because considerable swelling can occur.