When the outer side of the chest is visible through a gap in the clothing being worn by the subject, or if only the side of the chest is visible with, for example from behind angles (post #4805923, post #6728910, post #5224430).
If the nipples are visible, use nipple slip as well (post #4343645).
Do not confuse with pectoral cleavage.
This tag implicates pectorals (learn more).