Use 'immutable' to express that something is permanently unchanging and cannot be modified. The artist considered his final work immutable, refusing to alter a single stroke.
Pro Tip 2/3
Not Physically Bound
'Immutable' often refers to things that are not physically changeable, like rules or laws. The code of conduct for the knights was considered immutable.
Pro Tip 3/3
Misuse Warning
Avoid using 'immutable' for things that can change over time, even if they seldom do. Contrary to popular belief, social norms are not immutable.