A variation axis allows a continuous interpolation of outlines between at least two compatible sources (also called masters).
It is a trajectory between different states of a given shape. For example, the weight axis would shift points to adjust the stem thickness, allowing a transition from Thin to Bold styles.

FONT ENGINEERING HINT

The OpenType Specification defines 5 standard axes: weight, width, optical size, italic, and slant.
Note that italic acts as a stylistic link between two styles rather than a true interpolation axis.
These guidelines serve as a reference for how axes should be implemented into a font.

Variation axes are stored and referenced across several font tables:
• avar (Axis Variations): normalizes axis coordinates
• fvar (Font Variations): provides location coordinates for the instances
• STAT (Style Attributes): provides human-readable names for axes and their defined instances