CSS custom properties are commonly known as CSS variables, and judging by the main web development blogs and resources, most people likely use the terms “custom property” and “variable” interchangeably[.]
The reason for this could be that the CSS spec itself uses both terms. ”Custom properties” and “variables” both appear in the spec’s text dozens of times and even in the spec’s own title (“CSS Custom Properties for Cascading Variables”).
However, 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.
This distinction is useful because it allows us to talk about “variables with fallback values” (a custom property like any other property cannot have a fallback value) and “properties using variables” (a property cannot use a custom property) […] as well as “declaring a custom property on an element” (a variable isn’t declared but assigned to a property) and “the computed value of a custom property” (a variable doesn’t have a computed value but draws from the computed value of its associated custom property).