A missing link is a proposed, but as-yet undiscovered, intermediate form between two similar species, commonly used to refer to a form between man and a common ape-like ancestor.
Creationists frequently cite "Missing Links" as evidence that evolution is fallacious. Darwin called this issue "...the most obvious and gravest objection which can be urged against my theory..." and attributed the perceived lack of transitional fossils to imperfections in the geologic record.
First, the theory of evolution suggests that ALL life forms are "transitional forms". All fossils, then, are transitional fossils. Referring just to the line that lead to modern humanity, geologists have discovered hundreds of distinct, progressing variations.
Second, the nature of evolution suggests that the massive variation we find in life forms today are the direct descendants of a much less diverse group of life forms, possibly as small as a single individual form. While ducks and crocodiles do have common ancestors, their lines diverged long ago, and each line evolved independently from the other. Discovery of a "crocoduck" would be far more damaging to the theory of evolution than the absence of such a creature.