During this day in age, "the missing link" could be anything. But given your recent trend in questions, I am assuming you are refering to "the missing link" from human evolution. This missing link is/are examples of different evolutionary stages that bridge humans and their potential simian predecessors. And by examples, I mean fossil skeletons. We have already been able to unearth some potential previous relatives, but not very many. "The time of the split between humans and living apes used to be thought to have occurred 15 to 20 million years ago, or even up to 30 or 40 million years ago. Some apes occurring within that time period, such as Ramapithecus, used to be considered as hominids, and possible ancestors of humans. Later fossil finds indicated that Ramapithecus was more closely related to the orang-utan, and new biochemical evidence indicated that the last common ancestor of hominids and apes occurred between 5 and 10 million years ago, and probably in the lower end of that range (Lewin 1987). Ramapithecus therefore is no longer considered a hominid." (source: http://www.talkorigins.org/faqs/homs/species.html) Fossils from during this split are "the missing link" of human evolution. Alternatively there is rumor of something called the missing link of webpages, so if you want to learn more about good design practices with cascading style sheet please visit the following: http://www.uoguelph.ca/~stuartr/articles/missinglink.shtml

If I remember right, Darwin said at one point that as we unearth more and more fossils, we will find that the vast majority of historic life forms were transitional forms that will illuminate the map of how species differentiated over time. Darwinian evolution requires that intermediate forms must have existed and that the rise of complexity of species should be demonstrable from the incremental changes evident in the fossil records. Until there are fossil records of incremental change from some prototypical ancestor to a modern species, all of the "gaps" in the line are 'missing links.' If a scientist believes he has some skeletal evidence of a transition between any two species, it typically creates a new gap on either side of the transitional form. I don't think we currently have any continuous incremental records of any possible species transitions. I say "possible" transition because even if we had a proposed series of intermediate forms, we could be wrong. There have been many mis-identified partial skeletons in the history of evolution research, and our transitional forms could turn out to be statistical outliers within a population, or just unique mutations that did not survive... or something else (e.g. a mule conceivably could be mistaken for a transitional form, but mules can't reproduce). Science really can not test history - it does not have the tools. All the researcher can do is state that his hypothesis is consistent with some collection of historical evidence. While plausible or perhaps even very likely, any deduction about historical mutations or their ancestors and descendents is necessarily speculative, not hard science. I also think "the missing link" ought to be "the missing links" because there are so many holes in transition fossil records.

The two basic theories about how man and other species are what they are now are: "creationism" and "evolution". According to the first one species one way or another existed from the beginning more or less the same. According to Darwin's evolution, it all started with a single cell that evolved during the ages to many different species and sub-species that passed thru several different stages to become what the are today. Of course both theories have "holes" or dark spaces, in the specific case of evolution theory to be able to proof it, you should be able to find fossils of all the middle stages in the different layers of earth. For example, in the case of giraffes, Darwin states that they were not tall and with a long neck at the beginning, but more like let's say zebras, and being exposed to an environment with tall trees, their need to reach the higher branches made them grow long legged and long necked in time. The problem is that we have never found a middle-point-giraffe fossil, in other words an evolutionary stage between the old giraffe and the one we know these days. That would be the missing link for giraffes. In the case of humans we have many different types of apes: chimpanzees, gorillas, orangutans, etc. But we haven't found the middle-point-man between them and us. We have the australopitecus, the neandertal man, the ramaphitecus, etc., but some of them based on their physical features respond to be actual apes, or to close to humans (hominids). The finding of our "missing-link", as well as the "missing-links" for other species would proof then "evolution". Folklore has made myths like the Yeti, the Sasquatch, BigFoot, the abominable snowman and other very popular. They are all supposed to be this "missing" middle stage,