Impossible to overcome or defeat

unconquerable obstacles to success; an unconquerable faith.

not capable of being conquered or vanquished or overcome; "a tribute to his courage...and his unconquerable will"- R.E.Danielson; "faced unconquerable difficulties"[ant: conquerable ]

incapable of being surmounted or excelled; "insuperable odds"; "insuperable heroes"[syn: insuperable ]

adjective

impregnable , indomitable , indefeasible , inexpugnable , innate , insuperable , insurmountable , intractable , invincible , invulnerable , irrepressible , resistant , secure , unbeatable , unsurpassable , unyielding

noun

impregnability , invincibility