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]