The word "un-definable" is often used to describe something that cannot be defined, characterized, or explained thoroughly. Some of the antonyms for "un-definable" include words like clear, defined, distinct, evident, identifiable, and noticeable. These words denote that there is a definite shape, form, or structure to something that makes it easily categorizable or describable. Other antonyms for "un-definable" include words like apparent, definite, observable, determined, and recognizable. These words suggest that the object or concept being described has a clear purpose or function, and its attributes can be discerned or understood with ease.