An antonym is a word that has an opposite meaning to another word. In the case of the term 'un-defined,' the antonyms can be defined, clear, coherent, explicit, and distinct. These words all indicate a precise and well-understood meaning. 'Defined' is the word most commonly used as the antonym for 'un-defined' because it conveys the opposite meaning of being specific and having a clear meaning. A defined object, word, or concept is easily understood without any confusion or ambiguity. Therefore, it is best to use an antonym for 'un-defined' to provide clarity and understanding in communication.