noun
outside telephone line; external line
Refers to a telephone line that connects to outside numbers, as opposed to an internal extension (内線).
外線に出るには、最初にゼロを押してください。
To get an outside line, please press zero first.
外線が混み合っていて、なかなかつながらない。
The outside lines are busy and I can't get through.
noun
outer circle; outer line
Used in geometry or technical contexts for an external circle or line, such as in diagrams or sports fields.
この図形の外線の長さを計算してください。
Calculate the length of the outer line of this figure.