Axis

noun

1. 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"

2. 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)
3. The second cervical vertebra of the spine
4. A form of classification and descriptions of mental disorders or disabilities used in manuals such as the DSM (Diagnostic and Statistical Manual of Mental Disorders)
5. The main stem or central part about which organs or plant parts such as branches are arranged