Either a piercing of the center, lower lip or the jewelry that goes there. When referring to the jewelry itself, the labret is a barbell with a ball or decorative piece on one end and a flattened piece on the other so that it fits comfortably between the inside of the lip and the lower gum line.
A labret is one form of body piercing. Taken literally, it is any type of adornment that is attached to the facial lip (labrum). However, the term usually refers to a piercing that is below the bottom lip, above the chin.