Source : Webster's Revised Unabridged Dictionary (1913)

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.

         Lead forth to battle these my sons Invincible.
                                                  --Milton.
   -- {In*vin"ci*ble*ness}, n. -- {In*vin"ci*bly}, adv.

Source : WordNet®

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