A wisdom tooth or third molar is one of the three molars per quadrant of the human dentition. It is the most posterior (most distal) of the three. Wisdom teeth generally appear between the ages of 17 and 25. Most adults have four wisdom teeth (a third molar in each of the four quadrants), but it is possible to have fewer or more, in which case the extras are called supernumerary teeth. Wisdom teeth commonly affect other teeth as they develop, becoming impacted or "coming in sideways." They are often extracted when this occurs.

Anatomy
Main article: Human tooth

Variation
The formation of wisdom teeth differs by population, ranging from practically zero in Tasmanian Aborigines to nearly 100% in indigenous Mexicans. The difference is related to the PAX9 gene (and perhaps other genes).
Function

See also: Human vestigiality
Wisdom teeth are vestigial third molars that used to help human ancestors in grinding down plant tissue. The common postulation is that the skulls of human ancestors had larger jaws with more teeth, which were possibly used to help chew down foliage to compensate for a lack of ability to efficiently digest the cellulose that makes up a plant cell wall. As human diets changed, smaller jaws gradually evolved, yet the third molars, or "wisdom teeth", still commonly develop in human mouths.

Clinical significance
Wisdom teeth (often notated clinically as M3 for third molar) have long been identified as a source of problems and continue to be the most commonly impacted teeth in the human mouth. The oldest known impacted wisdom tooth belonged to a European woman of the Magdalenian period (18,000–10,000 BC). A lack of room to allow the teeth to erupt results in a risk of periodontal disease and caries that increases with age. Only a small minority (less than 2%) of adults age 65 years or older maintain the teeth without caries or periodontal disease and 13% maintain unimpacted wisdom teeth without caries or periodontal disease.

History
Although formally known as third molars, the common name is wisdom teeth because they appear so late – much later than the other teeth, at an age where people are presumably "wiser" than as a child, when the other teeth erupt.The term probably came as a translation of the Latin Latin: dens sapientiae