panty line       
noun [ C ] /ˈpæn.ti ˌlaɪn/ /ˈpæn.t̬i ˌlaɪn/
 内裤线
the edge (usually the bottom edge) of someone's pants (= underwear covering the area between the waist and the top of the legs), especially when it can be seen through their clothes
  Thongs have become a necessary fashion item for people who don't want panty lines to show.
 She says you can ruin an outfit with even a hint of a panty line. 她说一丝内裤线就足以毁掉一套衣服。 
 Compare: VPL
 - More examples
 - Magazines constantly warn against "unsightly panty lines" but personally, I don't care.
 - If you don't want a panty line, why wear panties at all?
 - Thongs may be uncomfortable, but how else do you fight visible panty line?