Bacterial vaginosis: A vaginal condition characterized by an abnormal vaginal discharge due to an overgrowth of normal bacteria in the vagina. Women with bacterial vaginosis also have fewer than the usual population of vaginal bacteria, called lactobacilli. Symptoms of bacterial vaginosis are vaginal discharge and sometimes a fishy odor. A microscopic sign of bacterial vaginosis is an unusual vaginal cell called a clue cell. Treatment options include oral antibiotics and vaginal gels. Bacterial vaginosis can cause premature labor and delivery, as well as infection of the amniotic fluid and of the uterus after delivery. Therefore, screening and treatment for bacterial vaginosis during pregnancy may be done.