Wedding vows and vault combinations that can’t be broken are considered inviolable. (Of course, divorce lawyers and bank robbers consider this a challenge.)
The word can refer to a physical structure (a fortress, for instance) or something more conceptual (human rights or morals, perhaps). Inviolable has changed little from its Latin origin of inviolabilis, which combines the prefix in- (meaning "not") with the verb violare ("to violate"). Inviolable turns up in religious settings too, usually in reference to texts or rites. In that context, it means "sacred." No surprise: the antonym of inviolable is violable ("accessible or penetrable").