| immemorial(lasting, permanent, everlasting, unending, undying) |
| in for the long haul(expression, idiom, stable, constant, enduring) |
| in the bag(expression, adjective, idiom, certain, limited) |
| inalterable(adjective, certain, limited, definite, inflexible) |
| inborn(fixed, persistent, continuous, lasting, continuing) |
| inbred(fixed, persistent, continuous, hardened, lasting) |
| incurable(fixed, persistent, continuous, hardened, lasting) |
| indefinite(lasting, permanent, everlasting, unending, undying) |
| indelible(permanent, long-term, lifelong) |
| indomitable(adjective, persevering) |
| ineradicable(fixed, persistent, continuous, hardened, lasting) |
| inexhaustible(certain, reliable, trustworthy, permanent, long-term) |
| inexpungible(permanent, long-term, lifelong) |
| inflexible(adjective, strong, flat, certain, determined) |
| ingrained(fixed, persistent, continuous, hardened, lasting) |
| intact |
| intense(adjective, firm, solid, steady, stable) |
| intent(adjective, strong, flat, certain, determined) |
| intransient |
| inviolate |