The Collaborative International Dictionary of English v.0.48:

Columella \Col`u*mel"la\, n. [L., dim. of columen column. See
   Column.]
   1. (Bot.)
      (a) An axis to which a carpel of a compound pistil may be
          attached, as in the case of the geranium; or which is
          left when a pod opens.
      (b) A columnlike axis in the capsules of mosses.
          [1913 Webster]

   2. (Anat.) A term applied to various columnlike parts; as,
      the columella, or epipterygoid bone, in the skull of many
      lizards; the columella of the ear, the bony or
      cartilaginous rod connecting the tympanic membrane with
      the internal ear.
      [1913 Webster]

   3. (Zool.)
      (a) The upright pillar in the axis of most univalve
          shells.
      (b) The central pillar or axis of the calicles of certain
          corals.
          [1913 Webster]

WordNet (r) 3.0 (2006):

columella
    n 1: a small column (or structure resembling a column) that is a
         part of a plant or animal