[TabAtkins]

Properly, the --foo stuff is a Custom Property. You can use its value as a variable, with var(), but it's not the only use custom properties have. That's why the spec's full name is "CSS Custom Properties for Cascading Variables". But that's a horrible mouthful, so "CSS Variables" is what it's usually called. (Plus that's the shortname in the url.)

(There's also a large focus, in v1 here, on the variables use-case, because that's what I needed to sell it to the group at the time. The CSS Houdini Task Force is now expanding Custom Properties properly, so they can actually be used for random JS things without a ton of hackery.)