únat (pl. únati) is a Quenya noun meaning "a thing impossible to be or to be done". The term refers to physical laws established by Ilúvatar that are impossible to break. The únati contrast with the axani, which are laws established by Ilúvatar that lesser beings should not, but can, disobey. Melkor refused to accept any axani as binding upon him, but much to his frustration, he was unable to violate the únati.
The word consists of the prefix ú-, meaning "not, un-, in- (usually with bad sense)", and nat, meaning "thing".