NOUN (5)

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;

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   3. (Anc. Armor) An iron breastplate, worn under the hauberk.
      [1913 Webster]

   3. (Anat.) The ventral shield or shell of tortoises and
      turtles. See Testudinata.
      [1913 Webster]

   4. A trimming for the front of a woman's dress, made of a
      different material, and narrowing from the shoulders to
      the waist.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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