Definition of "missing link" []

• A theoretical primate postulated to bridge the evolutionary gap between the anthropoid apes and humans. Not in scientific use. (noun)
• Something lacking that is needed to complete a series. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "I had long suspected that there had indeed been a connection and that the Knights Templar, through their discoveries during the Crusades, might have formed the missing link in the chain of transmission of secret architectural knowledge…St Bernard, the Templars' patron, had defined God—astonishingly for a Christian—as ‘length, width, height and depth’."
• "And Alobar intuited that it was the missing link in the evolution of the perfect perfume."
• "Diverge, and traces of a missing link lost in the sea that enlarged her form and stripped her fur and changed her contours -- and left cousins behind to revert to an earlier, more streamlined shape, but still airbreathing and milk-nursing."