When the bottom teeth sit forward of the top teeth. You can usually notice someone with an underbite by looking at their side profile. If their chin is protruding forward of their top lip, it is likely they have an underbite. An underbite is usually caused by inadequate growth of the upper jaw, overgrowth of the lower jaw, a combination of the two or by missing teeth.
For an example of an underbite correction at Gentle Touch Orthodontics please click here.