Axis refers to the initial sense of up/down and left/right. Think of axis as the spine of a figure -- the dominant vertical quality of a stick figure is determined by the central axis of the stick figure. The axis assists in the perception of depth, part of shapes, proportion, and ultimately what the shape actually represents.
An axis is generally medial.
An axis tends to ignore "limbs."
Master Concepts List - alphabetical glossary
Master Concepts List - grouped by category/course