"Missing link" is the term used to indicate a missing stage in the evolution of a species. The theory of evolution by natural selection claims that each species has evolved from another by insensible stages. Therefore, a continuous record of transitional forms should exist from the earlier form to the current form, each almost indistinguishable from those immediately before and after. Where such a transitional form is absent, it is referred to colloquially as a "missing link".
Missing transitional forms are a serious obstacle for the theory of evolution. Some 100 million fossils are stored in museums around the world, and yet no fossil has ever been found to be intermediate between two species. Since there are estimated to be about 100 million species in existence, and since evolutionists assert that a species last on average about 1 million years, given a history for life of just 200 million years, clearly there is at least one fossil for every 200 species. So why are the transitions between species not exhaustively recorded?
Instead, when a fossil cannot clearly be assigned to a known species, scientists generate a new species to accommodate the new specimen. This has the obvious consequence that two more missing links between species exist, where once there was one. As a result, since Darwin first proposed his theory, the number of missing links has actually grown. Where in his time there was a single missing link between fish and amphibians, now there are dozens. Where there was one alleged missing link between apes and humans, now there are at least twenty. Clearly the number of missing links is increasing with ever new "intermediate" discovery and Darwin's credibility is being degraded over time, reductio ad absurdum.
Clearly, eventually a set of "intermediates" will be found spanning the entire course of the "evolution" of a species from another, at which point it can be seen that the entire alleged sequence consists of missing links, and at no point will a species exist which can be seen not to belong unambiguously to a species, whether the first, last or some species name invented to accommodate an "intermediate" stage.