There is no exact definition of a gummy smile; however, it is generally recognised as when 3-4 millimetres of the gum line is exposed, leading to a noticeable amount of gum showing when smiling. This is often influenced by factors such as the shape and height of the teeth, the way the lips move and the angle of the jaw. Overactive muscles above the top lip can also contribute to a gummy smile.

The growth and positioning of the teeth can also lead to a gummy smile. For instance, a condition known as altered passive eruption can cause excessive gum coverage over the teeth. In other cases, gums might grow too far or experience over-eruption, a condition referred to as dentoalveolar extrusion.