Erythema nodosum is a medical condition characterized by painful, tender, and inflamed nodules or bumps under the skin. This condition is caused by an inflammation of the fat cells under the skin, which often occurs as a response to an infection, medication, or an underlying medical condition. Some of the synonyms for erythema nodosum include nodular erythema, subacute nodular migratory panniculitis, and septal panniculitis. Additionally, other types of erythema-related conditions may include erythema multiforme, erythema marginatum, and erythema elevatum diutinum. Although erythema nodosum is not a serious condition, it can be uncomfortable and painful, and should be evaluated by a medical professional to determine the underlying cause and appropriate treatment.