There are two different definitions, depending on the context.
In coordinate (or analytical) geometry an axis is one of one or more numbered straight lines. If there are more than one line, usually they are orthogonal - that is, they meet at 90 degree angles.
The second definition of an axis is in the context of symmetry. Given any shape, an axis is a straight line such that the shape is symmetric about that line. The shape may have 2 or more dimensions.
These two concepts are related in the sense that the equation of the shape usually becomes much simpler if its axis of symmetry is made into one of the coordinate axes. However, mathematicians knew about axes of symmetry long before coordinate geometry was developed.
to turn around on an axis
Of or pertaining to an axis; of the nature of, or resembling, an axis; around an axis., Belonging to the axis of the body; as, the axial skeleton; or to the axis of any appendage or organ; as, the axial bones.
The vertical axis is a reference axis usually denoted as y-axis in Cartesian co-ordinate system. It can be said that a vertical axis is the axis perpendicular to a reference horizontal surface.
x axis
longitudinal axis: An axis along the lengthwise direction of the figure or body, usually passing through its center of gravity.
dependent, y-axis, independent, x-axis, forgot the r and m
axis - An imaginary line that passes through earths center and its north and south poles.
Axis powers included Germany, Italy and Japan - in World War II.
Quadrature axis of a magnetomotive force is defined as that component of MMF that is directed along an axis in quadrature with the axis of the field poles. Quadrature axis of a magnetomotive force is defined as that component of MMF that is directed along an axis in quadrature with the axis of the field poles.
By definition, the circular cross-section of a cone changes linearly in width as you go along its axis. By definition, the cross-section of a prism is constant along its axis. So, by definition, a cone prism is an impossible shape.
The X axis is the horizontal(left-right) axis on a graph.
The bootom of the coordinate grid