i prefer calling this concept by its original name, "metafont", in part because "variable font" is sometimes used as lazy shorthand for "variable-pitch font", i.e. proportional rather than fixed-pitch like a typewriter. on the other hand, "metafont" is also the name of a particular piece of free software for this purpose
A good overview of variable fonts is "Introducing OpenType Variable Fonts" (2016): https://medium.com/variable-fonts/https-medium-com-tiro-intr...