noun · the limit of what one is prepared to accept

他的行为已经触及了我的道德底线。

noun · the line marking each end of a court in sports such as tennis or basketball

网球落在了底线附近。

noun · an undercover agent, informer, or plant within an organization

诚信经营是一个商人必须守住的底线。

noun · the lower thread in a sewing machine

缝纫机的底线断了，需要重新安装。