It's a seemingly rare case where the US terminology makes more sense than the rest of the world. In the US, a "fixed" mortgage is a fixed rate for the entire term of the loan (usually 30 years). Anything else is called an ARM (adjustable rate mortgage), even if it has a fixed component (e.g. a 5-year ARM has a fixed rate for 5 years and then becomes adjustable for the remainder of the term).

As I understand it, what the US considers a fixed rate mortgage simply doesn't exist in most countries, so it makes some amount of sense that the term "fixed" gets used for the next closest thing.