This tag is to be used when well-developed oblique muscles are noticeable, i.e. you should not use this tag to describe hip lines or hip bones. The external oblique muscle is one of the outermost abdominal muscles, extending from the lower half of the ribs around and down to the pelvis ending where the groin begins. Common names to describe developed obliques are "fish gills" (upper) and the Adonis or Apollo's Belt (lower). While it is possible for females to develop this muscle, it is more visible and far more common in males due to the differences in bone structure and distribution of body fat.
External Links
(front view)
Biology Dictionary: Oblique muscle (a lot of good info plus multiple images; male musculature)
