Class III Malocclusion occurs when your lower jaw protrudes forward, causing your lower teeth to overlap your upper teeth (also known as a severe underbite). In dental terminology, this condition is known as Prognathism (or Prognathia), from the Latin prefix Pro (meaning forward) plus the Latin word Gnathous (jaw).
Edward Angle, the father of modern orthodontics, was the first to classify malocclusion types. He based his classifications on the relative position of the top first molar to the other teeth. In cases where the first molars are missing, Orthodontists use canine teeth to determine the relationship.
Illustration courtesy of dentodontics.com