Mammary glands of the human female.
This tag is commonly used to draw attention to images that focus on the breast(s) via composition or rendering.
Most often used in reference to larger breasts, as size decreases this tag is less commonly used in favor of tags such as ちっぱい (small) or 貧乳 (small chest). For breasts that are significantly larger than average some tags such as 超乳 ("Super Milk") or 極乳 ("Extreme Milk") may also be used to denote the focus on a considerably large pair of breasts.
Since breasts are an important and common theme in artwork and life, many euphemisms and alternate tags are also used to further specify exactly what kind of appeal the artist is looking to capture in their rendition of the breasts.
Users will often honor particularly appealing images with tags such as 極上の乳 ("Incredible Breasts"). It is considered rude for an artist to add this tag to their own works, as its intended usage is for viewers to express their admiration and love for the artist's depiction of the breasts.