This article deals with the anatomy of wisdom teeth. For wisdom teeth removal surgery see: Impacted wisdom teeth
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.
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.
Wisdom teeth may have another function that has been researched in recent years, to be used as stem cells. The stem cells inside of a wisdom tooth recently removed from a patients mouth can be extracted and saved for a later use in brain cells, nerve cells, or to even grow another tooth. 
A wisdom tooth protrudes outwards from the gumline with inflamed tissue at the back (pericoronitis; green arrow)
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.
Some problems which may or may not occur with third molars: A Mesio-impacted, partially erupted mandibular third molar, BDental caries and periodontal defects associated with both the third and second molars, caused by food packing and poor access to oral hygiene methods, C Inflamed operculum covering partially erupted lower third molar, with accumulation of food debris and bacteria underneath, D The upper third molar has over-erupted due to lack of opposing tooth contact, and may start to traumatically occlude into the operculum over the lower third molar. Unopposed teeth are usually sharp because they have not been blunted by another tooth (attrition).
Impacted wisdom teeth are classified by the direction and depth of impaction, the amount of available space for tooth eruption. and the amount soft tissue or bone (or both) that covers them. The classification structure allows clinicians to estimate the probabilities of impaction, infections and complications associated with wisdom teeth removal. Wisdom teeth are also classified by the presence (or absence) of symptoms and disease.
Treatment of an erupted wisdom tooth is the same as any other tooth in the mouth. If impacted, treatment can be localized to the infected tissue overlying the impaction:440–441, extraction or coronectomy.
The upper left (picture right) and upper right (picture left) wisdom teeth are distoangularly impacted. The lower left wisdom tooth is horizontally impacted. The lower right wisdom tooth is vertically impacted (unidentifiable in orthopantomogram).
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: dens sapientiae.