noun /ˈæksɪs,ˈæksəs/
a) An imaginary line around which an object spins (an axis of rotation) or is symmetrically arranged (an axis of symmetry).
The Earth rotates once a day on its axis.
b) A fixed one-dimensional figure, such as a line or arc, with an origin and orientation and such that its points are in one-to-one correspondence with a set of numbers; an axis forms part of the basis of a space or is used to position and locate data in a graph (a coordinate axis).
Wikipedia foundation.