Source : Webster's Revised Unabridged Dictionary (1913)

Plastron \Plas"tron\, n. [F. plastron breastplate, plastron, LL.
   plastra a thin plate of metal. See {Plaster}.]
   1. A piece of leather stuffed or padded, worn by fencers to
      protect the breast. --Dryden.

   3. (Anc. Armor) An iron breastplate, worn under the hauberk.

   3. (Anat.) The ventral shield or shell of tortoises and
      turtles. See {Testudinata}.

   4. A trimming for the front of a woman's dress, made of a
      different material, and narrowing from the shoulders to
      the waist.

Source : WordNet®

plastron
     n 1: the ornamental front of a woman's bodice or shirt
     2: the front of man's dress shirt
     3: a metal breastplate that was worn under a coat of mail
     4: a large pad worn by a fencer to protect the chest
     5: (zoology) the part of a turtle's shell forming its underside