Noun

1. A kind of shield, of various shapes and sizes, worn on one of the arms (usually the left) for protecting the front of the body. In the sword and buckler play of the Middle Ages in England, the buckler was a small shield, used, not to cover the body, but to stop or parry blows.
   • 1598, William Shakespeare, Henry IV, Part I, Act II, Scene IV, line 166.

     I am eight times thrust through the doublet, four through the hose, my buckler cut through and through; my sword hacked like a hand-saw -- ecce signum!

2. A shield resembling the Roman scutum. In modern usage, a smaller variety of shield is usually implied by this term.
   • 1786, Francis Grose, A Treatise on Ancient Armour and Weapons, page 22:

     The target or buckler was carried by the heavy armed foot, it answered to the scutum of the Romans; its form was sometimes that of a rectangular parallelogram, but more commonly had it's bottom rounded off; it was generally convex, being curved in it's breadth.

3. One of the large, bony, external plates found on many ganoid fishes.
4. The anterior segment of the shell of trilobites.
5. A block of wood or plate of iron made to fit a hawse hole, or the circular opening in a half-port, to prevent water from entering when the vessel pitches.