axiom

Short definition from Wiktionary

Noun

1. 1
   A seemingly self-evident or necessary truth which is based on assumption; a principle or proposition which cannot actually be proved or disproved.
2. 2
   A fundamental assumption that serves as a basis for deduction of theorems; a postulate (sometimes distinguished from postulates as being universally applicable, whereas postulates are particular to a certain science or context).
3. 3
   An established principle in some artistic practice or science that is universally received.