Unchangeable or fixed.
Here's why:
* Immutable implies that something cannot be altered or modified after it's created.
* It emphasizes the state of being permanent and unchanging.
Other phrases that could be used, but might not be as precise, include:
* Unalterable
* Constant
* Permanent
However, unchangeable or fixed most accurately captures the core meaning of immutable, emphasizing its inherent resistance to change.