Any of various aquatic or terrestrial reptiles of the order Testudines (or Chelonia), having horny toothless jaws and a bony or leathery shell into which the head, limbs, and tail can be withdrawn in most species. Chiefly British A sea turtle. To hunt for turtles, especially as an occupation. Nautical To capsize. Archaic A turtledove. A turtleneck.