noun, measure word · a long, narrow mark, geometric path, or boundary

请在虚线上签名。

noun · a thin, flexible filament of material such as thread, string, or wire

这件衣服的线开了。

noun · an unofficial ranking category for Chinese cities, usually preceded by a number

上海是一线城市。

noun · a classifier used for a narrow beam or a tiny amount of something, such as light, hope, or life

在这绝望的时刻，他看到了一线生机。