Undecylenic refers to a type of organic compound that contains an undecyl group, which is a chain of 11 carbon atoms. It can also refer to a chemical compound with an undecylenic acid structure, which has the molecular formula C11H24O and is derived from castor oil. Undecylenic acid is known for its antifungal properties and is commonly used in various applications such as medicated shampoos, creams, and powders to treat skin conditions like athlete's foot or ringworm.