What is the "missing link"?
A missing link is fossil of an unique organism. Scientists regard such missing link as an evidence of evolutionary process that took place on earth.
Missing link is an organism of distant past which bridged a gap that we notice between related organisms which populate the earth today; e.g. between reptiles and bird or between apes and human.
The idea of a missing link as evidence of evolution definitely arose with the discovery of fossil of Archaeopteryx : in 1858/1859 Darwin proposed his Theory of Natural Selection, first by publishing a paper with Wallace, later in the form of a book. In 1861 this transitional form between lizard and bird was unveiled. It seemed to vindicate Darwin's idea of evolution.
Several other missing links are now known from fossil records. Australopithecus is the missing link between ape and man.