An axis is a component of a coordinate system. A prim can be moved in many ways along different axes while in edit mode. The 3 basic axes in Second Life are x (back/forward, horizontal), y (left/right, horizontal), and z (up/down, vertical). These 3 axes are used throughout the UI, such as in moving a prim, rotating a prim, and stretching a prim.
Axes are also sometimes used in scripting and will be represented by vectors or rotations, both of these are also types of variables. In scripting, a vector will look like "<x, y, z>" while a rotation will look like "<x, y, z, s>".
- LSL Wiki: LSL Wiki: axis