sagging breasts
When breasts have a noticeable sag to their shape, as opposed to the unnaturally perky breasts that are common in anime.
Not to be used for breasts that are simply hanging freely due to the character bending over or leaning forward.
Note that small breasts can also have a saggy appearance to them, although this is less common.
Examples
See also
This tag implicates breasts (learn more).