A poniard is a small knife with a narrow, sharp blade. If you'd been a knight during the Middle Ages, you probably would have carried a poniard.
A poniard, or poignard, is a type of dagger that you're only likely to see in a museum or used by a historical re-enactor. French and Italian Renaissance nobles were known to carry poniards, and to occasionally use them in hand-to-hand combat. The word is French, from the Old French poignal, used to mean "dagger," but literally "anything grasped in the fist," from poing, "fist."