Source : Webster's Revised Unabridged Dictionary (1913)

Rampant \Ramp"ant\, a. [F., p. pr. of ramper to creep. See
   {Ramp}, v.]
   1. Ramping; leaping; springing; rearing upon the hind legs;
      hence, raging; furious.

            The fierce lion in his kind Which goeth rampant
            after his prey.                       --Gower.

            [The] lion . . . rampant shakes his brinded mane.
                                                  --Milton.

   2. Ascending; climbing; rank in growth; exuberant.

            The rampant stalk is of unusual altitude. --I.
                                                  Taylor.

   3. (Her.) Rising with fore paws in the air as if attacking;
      -- said of a beast of prey, especially a lion. The right
      fore leg and right hind leg should be raised higher than
      the left.

   {Rampant arch}.
      (a) An arch which has one abutment higher than the other.
      (b) Same as {Rampant vault}, below.

   {Rampant gardant} (Her.), rampant, but with the face turned
      to the front.

   {Rampant regardant}, rampant, but looking backward.

   {Rampant vault} (Arch.), a continuous wagon vault, or cradle
      vault, whose two abutments are located on an inclined
      planed plane, such as the vault supporting a stairway, or
      forming the ceiling of a stairway.

Source : WordNet®

rampant arch
     n : an arch whose support is higher on one side than on the
         other