Noun
1. The nearly flat part of the shell structure of a turtle, what one would call the belly or ventral surface of the shell. It also includes within its structure the anterior and posterior bride struts and the bridge of the shell. (wikipedia.org)
2. The ventral part of the shell of a tortoise or turtle consisting typically of nine symmetrically placed bones overlaid by horny plates. (merriam-webster.com)
3. The part of a turtle’s shell forming its underside. (wordnetweb.princeton.edu)
Word origin: From Middle French plastron “breastplate,” from Italian piastrone, augmentative of piastra “breastplate, thin metal plate”