Wisdom teeth
Wisdom teeth are third molars that usually appear between the ages of 18 and 20 (although they may appear when older). They are called "wisdom teeth" because they appear so late—much later than the other teeth, at an age where people are supposedly wiser than as a child, when the other teeth erupt. Often they need to be removed when they impact against other teeth—colloquially known as "coming in sideways."
Impacted wisdom teeth fall into one of several categories. Mesial impaction means the tooth is angled forward, towards the front of the mouth. Distal impaction means the tooth is angled backward, towards the rear of the mouth. Vertical impaction occurs when the formed tooth does not erupt fully through the gumline, and horizontal impaction occurs when the tooth is angled fully ninety degrees forward, growing into the roots of the second molar.
Impacted wisdom teeth may also be categorized on whether they are still completely encased in the jawbone. If it is completely encased in the jawbone, it is a bony impaction. If the widsom tooth has erupted out of the jawbone but not through the gumline, it is called a soft tissue impaction.
Wisdom teeth are sometimes described as an example of a "vestigial" trait that may be disappearing from our species via evolution. Alternately, it is possible that wisdom teeth were useful when it was common for humans to have lost several teeth to decay by the age when they appear.
