used to refer to lines or distances east or west of an imaginary line between the North Pole and the South Pole

Longitudinal lines run from north to south. 经线的方向是由北至南。

in the long direction of the body or of any body part

a longitudinal rash 纵向皮疹

Longitudinal research is done on people or groups over a long period of time.

longitudinal analysis/studies 纵向分析／研究