WordWeb Online

1. Not capable of being changed or altered
   "an unalterable ground rule"; "unalterable resolve";
   - inalterable
    
2. (law) of a sentence; that cannot be changed
   "an unalterable death sentence"
    
3. Remaining the same for indefinitely long times
   - changeless

See also: final, incommutable, incurable, last, unchangeable

Antonym: alterable