wisdom tooth

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One of four rearmost molars on each side of the upper and lower jaw in humans. Wisdom teeth are the last teeth to erupt, typically in early adulthood.

Century Dictionary and Cyclopedia

1. n. The last molar tooth on either side of each jaw. It appears ordinarily between the ages of 20 and 25, presumably years of discretion (whence the name). Also technically called dens sapientiæ. Also wit-tooth.

Wiktionary

1. n. dentistry One of the four (one upper and one lower on each side) rearmost molars in humans, which typically develop between ages 18-24.

GNU Webster's 1913

1. n. the last, or back, tooth of the full set on each half of each jaw in man; -- familiarly so called, because appearing comparatively late, after the person may be supposed to have arrived at the age of wisdom. See the Note under Tooth, 1.

WordNet 3.0

1. n. any of the last 4 teeth on each side of the upper and lower jaw; the last of the permanent teeth to erupt (between ages 16 and 21)