noun · the mandibular joint or the physical area where the upper and lower teeth meet, often mentioned in the context of clenching or involuntary movement

由于极度寒冷，他的牙关不停地打战。

the clenching of the teeth as a figurative representation of determination, grit, or the endurance of pain and hardship

面对重重困难，他咬紧牙关，始终没有放弃。