Underboob

[Rihanna]

cleavage visible from the part of the shirt that covers the bottom of the female breasts

[Becky]

Hands down, one of the finest aspects of a womans anatomy that can be enjoyed in any public venue.
Underboob is achieved by wearing a very short halter top or cropped tank, also known as an [underboob shirt], which exposes the bottom areas of a womans breasts.
Much like the combination of low-rise jeans and a thong, the underboob shirt can only successfully be worn by select few women.

[Sonia]

Underboob: The only time in life when gravity is your friend.

[Sarah]

better than cleavage.

[Couch Potatoe]

Generally, when a women wears a short cut top/pulls up their top to cover all but a portion of the bottom of the breast. Nipple can be shown, but often it is also covered.
Other ways to achieve underboob are: [Handbra] (covering the breast in the same way a short cut top would), or viewing from a low angle so that the point of view is looking up at the womans breasts.