a device that can be put inside the mouth in order to protect the teeth and gums, for example when playing sport

Mouth guards prevent more than 200,000 injuries annually among high school and college athletes, the American Dental Association estimates. 据美国牙科协会估计，护齿套每年在高中和大学运动员中可防止20多万次受伤。

My dentist has given me a mouthguard to stop me grinding my teeth in my sleep.