Cradle cap: A form of seborrheic dermatitis. of the scalp in infants. It is characterized by flaking or scaling of the skin, The skin may also be reddened. Although cradle cap is on the scalp, the same process can involve the skin on the nose, eyebrows, scalp, ears, and skin of the trunk (particularly in skin folds). Seborrheic dermatitis is an inflammatory skin rash due to overactivity of the sebaceous glands in the skin.