noun · a military position or line of troops encircling an enemy position

我军在这次战役中处于外线作战的有利地位。

noun · a telephone line that connects to an external network or outside party

办公室的电话需要先拨0才能打通外线。

noun · the area of a basketball court located outside the three-point line

对方球队的外线防守非常严密。