noun
1. : an absent thing or member needed to complete a series
modern technology and capital are the only major missing links in the underdeveloped areas — J.K.Rose
2. : a hypothetical intermediate form between man and his presumed simian progenitors
supposed by some people to be the nearest thing to the missing link, and thus the most primitive human being in existence — Alan Moorehead
— compare ape-man , prehominid