Patient-friendly definition from the DentistsDirectory.org dental glossary.
Malocclusion (from Latin: "bad bite") describes any deviation from the ideal alignment of the upper and lower teeth when the jaws are closed together. It encompasses a wide spectrum from minor crowding or slight overbite to significant skeletal discrepancies between the upper and lower jaws. Malocclusion is extremely common — studies suggest the majority of the population has some degree of it, though only a fraction require treatment.
The most common classifications are based on the relationship of the upper and lower molars and canines. Class I malocclusion has a correct molar relationship but teeth that are crowded, spaced, or rotated. Class II (overbite/overjet) has the upper jaw or teeth protruding ahead of the lower — the stereotypical "buck teeth" appearance. Class III (underbite) has the lower jaw jutting ahead of the upper, causing the lower teeth to be in front of the upper when biting. Other patterns include crossbite (upper teeth sitting inside the lower on one or both sides), open bite (front teeth don't close together even when the back teeth are touching), and deep bite (upper front teeth excessively overlap the lower).
Mild malocclusion may have no functional impact. More significant malocclusion can make chewing inefficient, cause uneven tooth wear, contribute to jaw pain and TMJ problems, make cleaning difficult (leading to higher decay rates in crowded areas), and affect speech and appearance. Treatment depends on severity and the patient's age. Orthodontic treatment (braces or clear aligners) corrects dental malocclusion effectively. Significant skeletal discrepancies — where the jaws themselves are mispositioned rather than just the teeth — may require orthognathic (jaw) surgery combined with orthodontics.