In placental mammals, the umbilical cord is a tube that connects a developing embryo or fetus to its placenta. It contains major arteries and veins (notably the umbilical arteries and umbilical vein) for the exchange of nutrient- and oxygen-rich blood between the embryo and placenta. When the animal is born, the umbilical cord is severed and leaves only a small scar (the umbilicus) behind.
The umbilical cord develops from, and contains, remnants of the yolk sac and allantois.
In humans, the umbilical cord in a full term fetus is usually about 50 cm long and about 2 cm in diameter. It contains two umbilical arteries and one umbilical vein, buried within Wharton's jelly.
Recently, it has been discovered that the matrix within the umbilical cord (known as Wharton's jelly) is a rich and readily available source of primitive stem cells. Some parents have opted to have these stem cells harvested upon the baby's birth, and frozen for long-term storage should the child ever require them (for example to replace bone marrow destroyed when treating leukemia).
For more information on umbilical cord blood storage and transplant: umbilical cord blood bank.
Other uses for the term "umbilical cord"
The term "umbilical cord" or just "umbilical" has also come to be used for other cords with similar functions, such as the hose connecting a surface-supplied diver to his surface supply of air and/or heating, or a space-suited astronaut to his spacecraft.
The phrase "cutting the umbilical cord" is used symbolically to describe a child's breaking away from the parental home.