Urticaria (from the Latin urtica, "nettle" from urere, "to burn"), commonly referred to as hives, is a kind of skin rash notable for pale red, raised, itchy bumps. Hives may cause a burning or stinging sensation. They are frequently caused by allergic reactions; however, there are many nonallergic causes. Most cases of hives lasting less than six weeks (acute urticaria) are the result of an allergic trigger. Chronic urticaria (hives lasting longer than six weeks) is rarely due to an allergy. The majority of chronic hives cases have an unknown (idiopathic) cause. In perhaps as many as 30–40% of patients with chronic idiopathic urticaria, it is caused by an autoimmune reaction.