to make something longer, or to become longer

I'll have to lengthen this skirt. 我必须把这条裙子放长一些。

lengthening waiting lists 越来越长的等候者名单

If you lengthen something, or it lengthens, it takes longer to happen.

There is a plan to lengthen the three-year course to four years. 计划要将3年的课程延长为4年。