Discoid eczema affecting the upper arm. Eczema is a general term used to describe an inflammation of the skin that appears to arise without any obvious external causes and is often referred to as endogenous or constitutional eczema. Endogenous eczema is classified according to its appearance situation; the discoid type is characterized by small, well-defined areas. Skin reactions caused by external factors are termed eczematous dermatitis or simply dermatitis.