See: http://mathworld.wolfram.com/Axiom.html

An axiom is a statement taken as true without proof or supporting arguments.

Divmod Axiom is so named because it is a database, and a database is where you
put assertions about the world.  In particular a database is where you put
values which you do not wish to re-calculate; the data that your computation is
based upon.  In this way axiom items are similar to axioms, since (for example)
euclidean geometry can be derived from the set axioms known as "euclid's
postulates", but those axioms need to be stored independently; they cannot be
derived from anything.

Plus it has an X in it, which sounds neat.