The Collaborative International Dictionary of English v.0.48:

Rampant \Ramp"ant\ (r[a^]mp"ant), a. [F., p. pr. of ramper to
   creep. See Ramp, v.]
   1. Ramping; leaping; springing; rearing upon the hind legs;
      hence, raging; furious.
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            The fierce lion in his kind
            Which goeth rampant after his prey.   --Gower.
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            [The] lion . . . rampant shakes his brinded mane.
                                                  --Milton.
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   2. Ascending; climbing; rank in growth; exuberant.
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            The rampant stalk is of unusual altitude. --I.
                                                  Taylor.
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   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.
      [1913 Webster]

   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
      plane, such as the vault supporting a stairway, or forming
      the ceiling of a stairway.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rampant arch
    n 1: an arch whose support is higher on one side than on the
         other