Source : Webster's Revised Unabridged Dictionary (1913)

Jacob \Ja"cob\, n. [Cf. F. Jacob. See 2d {Jack}.]
   A Hebrew patriarch (son of Isaac, and ancestor of the Jews),
   who in a vision saw a ladder reaching up to heaven (--Gen.
   xxviii. 12); -- also called {Israel}.

         And Jacob said . . . with my staff I passed over this
         Jordan, and now I am become two bands.   --Gen. xxxii.
                                                  9, 10.

         Thy name shall be called no more Jacob, but Israel.
                                                  --Gen. xxxii.
                                                  28.

   {Jacob's ladder}.
   (a) (Bot.) A perennial herb of the genus {Polemonium} ({P.
       c[oe]ruleum), having corymbs of drooping flowers, usually
       blue. Gray}.
   (b) (Naut.) A rope ladder, with wooden steps, for going
       aloft. --R. H. Dana, Jr.
   (c) (Naut.) A succession of short cracks in a defective spar.
       

   {Jacob's membrane}. See {Retina}.

   {Jacob's staff}.
   (a) A name given to many forms of staff or weapon, especially
       in the Middle Ages; a pilgrim's staff. [Obs.] --Spenser.
   (b) (Surveying) See under {Staff}.

Source : WordNet®

Jacob's ladder
     n 1: pinnate-leaved European perennial having bright blue or
          white flowers [syn: {Greek valerian}, {charity}, {Polemonium
          caeruleum}, {Polemonium van-bruntiae}, {Polymonium
          caeruleum van-bruntiae}]
     2: (nautical) a hanging ladder of ropes or chains supporting
        wooden or metal rungs or steps [syn: {jack ladder}, {pilot
        ladder}]