To elaborate slightly, there's three components here:
let x = 5;
let mut y = 6;
The let statement binds a variable to a value:
* x and y are the variables
* 5 and 6 are values
* let does the binding
You can have an immutable binding, like x, or a mutable binding, like y. But most people turn "a bound variable" into "a variable" (or "an immutable variable") and "a mutably bound variable" into a "mutable variable".
"Immutable variables" is frequently used, true, but the official term is "immutable bindings".