(logic) a proposition that is accepted as true in order to provide a basis for logical reasoning WordNet 3.0 & Open English WordNet

Something that is posited; a postulate. Wiktionary

Postulate. Wiktionary (translation)

Something assumed without proof as being self-evident or generally accepted, especially when used as a basis for an argument. Sometimes distinguished from axioms as being relevant to a particular science or context, rather than universally true, and following from other axioms rather than being an absolute assumption. Wiktionary

Something assumed without proof as being self-evident or generally accepted. Wiktionary (translation)