The Collaborative International Dictionary of English v.0.48:

Invincible \In*vin"ci*ble\, a. [L. invincibilis: cf. F.
   invincible. See In- not, and Vincible.]
   Incapable of being conquered, overcome, or subdued;
   unconquerable; insuperable; as, an invincible army, or
   obstacle.
   [1913 Webster]

         Lead forth to battle these my sons
         Invincible.                              --Milton.
   -- In*vin"ci*ble*ness, n. -- In*vin"ci*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

invincible
    adj 1: incapable of being overcome or subdued; "an invincible
           army"; "her invincible spirit" [syn: invincible,
           unbeatable, unvanquishable]

Moby Thesaurus II by Grady Ward, 1.0:

148 Moby Thesaurus words for "invincible":
   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