When mentioning CSS Custom Properties here on bram.us I do tend to name them like that — and not CSS Variables — as that’s their official name.
I always thought the terms could be used interchangeably — with CSS Variables simply being the unofficial name — but as detailed by Šime Vidas on Web Platform News that’s not the case:
The spec distinguishes the two terms: A custom property is not a variable, but it defines a variable. Any property can use variables with the
var()function whose values are defined by their associated custom properties.