PLANTIGRADE is a 11 letter word that starts with P.

1. \Plan"ti*grade\, a. [L. planta sole of the foot +
   gradi to walk: cf. F. plantigrade.] (Zo["o]l.)
   (a) Walking on the sole of the foot; pertaining to the
       plantigrades.
   (b) Having the foot so formed that the heel touches the
       ground when the leg is upright.
   
   

2. \Plan"ti*grade\, n. (Zo["o]l.)
   A plantigrade animal, or one that walks or steps on the sole
   of the foot, as man, and the bears.