A statement that is accepted as true (without needing proof) so that further reasoning can be done.
Example: one of Euclid's axioms (over 2300 years ago) is: "Things equal to the same thing are equal to each other", in other words If A = B, and B = C, then A = C
Example: Another of Euclid’s axioms is If A = B and C = D, then A + C = B + D
Axioms are like the rules of a game, they’re agreed on at the start so everyone can follow the same logic.