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.

• The nearly flat part of the shell structure of a tortoise, similar in composition to the carapace;
  
• (fencing) A half-jacket worn under the jacket for padding or for safety.