Meaning of missing link

miss'ing link'

Pronunciation: [key]
  1. a hypothetical form of animal assumed to have constituted a connecting link between the anthropoid apes and humans, identified by some authorities as constituting the genus Australopithecus.
  2. something lacking for the completion of a series or sequence.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.