Definition from Wiktionary, the free dictionary
immutable (not comparable)
- Unable to be changed without exception.
- The government has enacted an immutable law.
- (programming, of a variable) Not able to be altered in the memory after its value is set initially, such as a constant.
Derived terms 
unable to be changed without exception
immutable (plural immutables)
- Something that cannot be changed.
immutable (masculine and feminine, plural immutables)