The term "missing link" is terribly misleading, yet it is used so often that it is difficult to ignore. When used with caution, it can communicate the context of a particular fossil find. But when used to imply that the fossil record is a long chain of related organisms, the term missing link is guaranteed to make palaeontologists cringe.
To understand why the term missing link is so often misused, let's first consider the fossil record and what it tells us about past life forms. Every fossil that is discovered was once a living organism. That organism was one individual within a population—one organism within generations of populations. A fossil is a pinpoint of evidence at the tip of an evolutionary branch, it's like a leaf on a tree. A fossil is an end point, not a link in a chain.
We have no way of knowing whether any single fossil is the direct ancestor of any other fossil or living animal. The fossil record offers us no proof of direct ancestory. Evolutionary trees are not genealogies. Their stems and branches represent relationships between entire groups of animals, or clades, not relationships between individuals.
So a single fossil is not a link in a chain. It represents one of many individuals of its kind and its significance comes from an analysis of how it resembles and differs from its nearest relatives on the evolutionary tree. A single fossil can never fully bridge any gap in the evolutionary tree.
The reality is that there are gaps in the evolutionary tree, some bigger than others. When fossils are discovered that help to reduce such gaps, they are better be described as "transitional fossils", not "missing links". But if the term "missing link" is to be used, it should be made clear that the term refers to a fossil that helps us to better describe the transition between ancestral organisms, not any direct link they share.