axis

noun

• 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"

• 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)
• The second cervical vertebra of the spine
• 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)
• The main stem or central part about which organs or plant parts such as branches are arranged