If one wants to classify all fonts as either monospace (fixed-width) or proportional (variable-width), then how should this property of a font be called?
Everything that I've ever seen which deals with fonts assumes that they are proportional width fonts unless they are called "monospace" fonts, and without resorting to technical terms that only font creators are likely to know, there isn't a common term for that property.
So for the sake of clarity I would call them "monospace" and "non-monospace" fonts in the same spirit of "serif" and "sans-serif" where "sans-serif" literally means "without serif".
If you need to list the property in a table or something similar, I would call the property "monospace" and label it as true/false, yes/no, or ticked/unticked whichever best matches the style of the rest of the table.
EDITED - Correcting myself, I had suggested that property could be letter-spacing or Kerning but its wrong. Letter-Spacing and Kerning represent spacing between font-characters and not width of characters which is being asked here.