A prominent, visible panty line or VPL (visible panty line) occurs when the outline of one's underpants becomes visible through one's clothes. There may be a ridge or depression in the clothes, or the clothes may be slightly see-through.
The taboo in many cultures against showing one's underwear means that panty lines are generally considered undesirable and embarrassing. At the same time, this also makes VPL an attribute of erotic clothing. Its erotic value leads to women and fashion designers paying a lot of attention to lingerie that will be worn under a see-through garment.
The origin of the term is probably traced to barracks humor, the term VPL for 'visible panty line' as a mockery of overused bureaucratic initialisms, coined in an analogy with water line.
Some lingerie companies advertise "invisible panty lines" as a selling point for, e.g., boy shorts and C-strings.
