Urticaria (from the Latin urtica, nettle (whence It. ortica, Sp. ortiga, Pg. urtiga, Fr. ortie) urere, to burn) (or hives) is a kind of skin rash notable for pale red, raised, itchy bumps. Hives is frequently caused by allergic reactions; however, there are many non-allergic 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 patients with chronic hives have an unknown (idiopathic) cause. Perhaps as many as 30–40% of patients with chronic idiopathic urticaria will, in fact, have an autoimmune cause. Acute viral infection is another common cause of acute urticaria (viral exanthem). Less common causes of hives include friction, pressure, temperature extremes, exercise, and sunlight.