Urachus: A canal that connects the urinary bladder to the umbilicus (bellybutton) during fetal development. The urachus is normally obliterated, so it is usually a solid cord. Failure for the urachus to fill in leaves it open. The telltale sign of an open urachus is leakage of urine through the umbilicus. An open urachus is a malformation and needs to be surgically corrected.