Buckle/ˈbʌkl/ Definition 1 of 4Pro Tip 1/2Safety First Often used in the context of safety measures, especially in vehicles. Always buckle your helmet securely before riding. Pro Tip 2/2Not Just Belts 'Buckle' is used for various fastening contexts, not limited to belts. Buckle the lifejacket's straps to ensure it stays on.