We chose Permanent because it means long-lasting or durable, but it has a hidden geeky meaning we liked: computing the permanent of a matrix is a #P-complete problem.
'continuing or enduring without fundamental or marked change'
google gives a similar
'lasting or intended to last or remain unchanged indefinitely.'
Both sites list a bunch of synonyms that are closer to long-lasting and durable, but english is subtle. One can't usually swap in a 'synonym' without changing meaning.