Short for the grouping of the abdominal muscles (sometimes also including Obliques).
The Abdominals are a paired muscle that takes the form of 2 long narrow sheets that extend down the full length of the abdomen converging on the groin. Divided vertically by the Linea Alba the abdominals are additionally devided horizontally by 3(sometimes more) bands of connective tissue extending from the obliques that give the abdominals it's characteristic six pack look in very fit people, the navel is usually intersected by the lowest band of the 3.
The abdominals chiefly support the erect upright gait of the body together with the back muscles and are responsible for forward mobility of the spine and torso in general. They also protect the organs of the abdomen as well as applying internal pressure on the organs for assisting in breathing as well as defecation or childbirth.

If the abdominals are well developed and visible in a picture then it should be tagged.