This is a situation in which one twin fails to develop past the fetal stage and is completely subsumed into the body of the other. Generally, it's fetus in fetu if the fetus develops a reasonable skeleton, but sometimes there are just bits and pieces of fetus bodies that are subsumed into otherwise healthy babies.
Fetus in fetu is a surgico pathological curiosity, wherein a vertebrate fetus is included within the abdomen of its partner. Masses containing bones, cartilage, teeth, central nervous system tissue, fat and muscle may be found in the abdomen of newborns and children termed Teratomas. They are defined as fetus in fetu if there is a recognized trunk and limbs, seemingly an abortive twinning.
Another description states that fetus in fetu (or foetus in foeto) describes an extremely rare abnormality that involves a fetus getting trapped inside its twin. It continues to survive as a parasite even past birth by forming an umbilical cord-like structure that leeches its twin's blood supply until it grows so large that it starts to harm the host, at which point doctors usually intervene.
Invariably the parasitic fetus is anencephalic (without a brain) and lacks internal organs, and as such is unable to survive on its own, though it may have almost human (albeit underdeveloped and bizarre) features such as limbs, digits, hair, nails and teeth.
Fetus in fetu was coined by Johann Friedrich Meckel in the early nineteenth century. It is an extremely rare condition estimated to occur once in 500,000 deliveries.