Well maybe global constants shouldn't be called "variables", but I don't see how your definition excludes local immutable variables from being called "variables". E.g.

  fn sin(x: f64) -> f64 {
    let x2 = x / PI;
    ...

Is x2 not variable? It's value varies depending on how I assign x.

Anyway this is kind of pointless arguing. We use the word "variable". It's fine.