Difference between "axes" and "axis" in matplotlib?

in the context of matplotlib,

axes is not the plural form of axis, it actually denotes the plotting area, including all axis.

Axis is the axis of the plot, the thing that gets ticks and tick labels. The axes is the area your plot appears in.

This figure from the documentation will answer your question:

You can find this image here (in the Matplotlib 1.x docs); it's actually been replaced in the Matplotlib 2.x docs.