thesaurus.maths.org

Something that we assume is true in order to work out a particular branch of maths, but doesn't have to be true; we could work out a different, parallel branch of maths by assuming something different. For example, Euclid's axioms include the 'parallel postulate' which is true if we want to do Euclidean geometry, but false if we want to do non-Euclidean geometry.