Mucus plug: An accumulation of mucus and thick fluid that blocks the opening of the cervix (the opening from the uterus to the vagina) during pregnancy. The mucus plug serves to protect the uterus and the fetus by preventing bacteria from entering the uterus. The mucus plus can be clear or slightly pink in color and can be tinged with brown or red blood. As the cervix slowly begins to dilate prior to the onset of labor, the mucus plug is expelled. When expelled, the mucus plug has been referred to as bloody show.. When the mucus plug is expelled, it may be noticed by the woman as a solid mass or as increased vaginal discharge over a few days.