Drift refers to gradual movement of the teeth:

• Toward an empty space between the teeth. Drift commonly occurs after the loss of a tooth.
• From the back to the front of the mouth. Drift is a normal process during the adult years. It seems to explain why many adults have increasingly crowded lower front incisors.

Drift may cause malocclusion (poor bite) and could require orthodontic treatment with braces or other devices.