not changing, or unable to be changed (Cambridge)
that cannot be changed; that will never change (Oxford)
ناقابل تغیر, بلا تبدیل,
This decision should not be seen as immutable. (Oxford)
Some people regard grammar as an immutable set of rules.(Cambridge)
The laws of physics are assumed to be immutable. (Cambridge)
An immutable law
Immutable set of rules
Unchangeable
Fixed
Rigid
Variable
Immutability
Immutable