A rondel dagger was a small dagger that was used particularly for stabbing through chain mail and through the joints of plate mail or eye-holes of a visor. The blade, while traditionally edged on both sides, was very thick, and cutting or slicing would not be its primary task. Some rondels gave up all pretense of being a cutting weapon, and were essentially a spike with a triangular or cruciform cross section.
The rondel was in use in Europe in the late Middle Ages (1300s onwards), and were used for sparring, and as a secondary sidearm. It came to replace the earlier knightly dagger, and by the 1400s was becoming a usual part of a knight's armament. They foreshadowed the eventual development of the Italian stiletto, which would eventually replace them.
The name 'rondel' (or sometimes 'roundel') comes from the traditionally flat, round hand guards and pommels. These daggers were sometimes also called a 'misericorde', from the Latin misericordia, literally meaning 'act of mercy'; this derived from the use of the rondel as an armor-piercing death-blow, which might also be used to put a severely wounded knight out of their misery.