(a.) Incapable of being conquered, overcome, or subdued; unconquerable; insuperable; as, an invincible army\, or obstacle.

assiduous beyond compare beyond comparison calculable constant continuing deathless dependable diligent dogged easily first enduring facile princeps faithful faithworthy fast firm immortal immutable impassable impenetrable imperishable impregnable inalterable incomparable incontestable incorruptible indefatigable indelible indestructible indomitable industrious ineffaceable ineradicable inexpugnable inextinguishable inimitable insistent insuperable insurmountable inviolable invulnerable irresistible lasting loyal matchless never-tiring nonperishable nulli secundus obstinate overpowering overwhelming patient patient as Job peerless permanent perseverant persevering persistent persisting pertinacious plodding plugging predictable preoccupied quenchless rapt relentless reliable resistless resolute second to none secure sedulous single-minded sleepless slogging solid sound stable staunch steadfast steady stubborn substantial sui generis sure surefire tenacious tireless trustworthy trusty unabating unapproachable unapproached unassailable unattackable unbeatable unconquerable undampable undaunted undefeatable undefeated undestroyable undiscouraged undrooping undying unequaled unexampled unexcelled unfailing unfaltering unflagging unflinching unintermitting uninterrupted unique unmatchable unmatched unnodding unparagoned unparalleled unpeered unquenchable unrelaxing unrelenting unremitting unrivaled unsleeping unstoppable unsubduable unsurmountable unsurpassable unsurpassed unswerving untiring untouchable unwavering unwearied unwearying unwinking unyielding utterly attentive weariless well-founded well-grounded without equal without parallel