The Collaborative International Dictionary of English v.0.48:

Ventral \Ven"tral\, a. [L. ventralis, fr. venter the belly;
   perhaps akin to G. wanst: cf. F. ventral.]
   1. (Anat.) Of, pertaining to, or situated near, the belly, or
      ventral side, of an animal or of one of its parts; hemal;
      abdominal; as, the ventral fin of a fish; the ventral root
      of a spinal nerve; -- opposed to dorsal.
      [1913 Webster]

   2. (Bot.)
      (a) Of or pertaining to that surface of a carpel, petal,
          etc., which faces toward the center of a flower.
      (b) Of or pertaining to the lower side or surface of a
          creeping moss or other low flowerless plant. Opposed
          to dorsal.
          [1913 Webster]

   Ventral fins (Zool.), the posterior pair of fins of a fish.
      They are often situated beneath the belly, but sometimes
      beneath the throat.

   Ventral segment. (Acoustics) See Loop, n., 5.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ventral
    adj 1: toward or on or near the belly (front of a primate or
           lower surface of a lower animal); "the ventral aspect of
           the human body"; "the liver is somewhat ventral in
           position"; "ventral (or pelvic) fins correspond to the
           hind limbs of a quadruped" [ant: dorsal]
    2: nearest to or facing toward the axis of an organ or organism;
       "the upper side of a leaf is known as the adaxial surface"
       [syn: adaxial, ventral] [ant: abaxial, dorsal]