When, in a resting position, the anterior or lateral positions of the tongue contact more than half the surface area of either the upper or lower incisors, cuspids or bicuspids or protrude between them. Or when, during the swallow of any two of three media, there is a visible increase by the tongue forced against the teeth, protrudes between the teeth or contacts the surface area of the teeth.

Tongue thrust (also called "reverse" or "immature" swallow) is the common name given to orofacial muscular imbalance, a human behavioural pattern in which the tongue protrudes through the anterior incisors during swallowing, speech and while the tongue is at rest. Nearly all young children exhibit a swallowing pattern involving tongue protrusion, but by the age of 6 most have automatically switched to a normal swallowing pattern.