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A '''transitional fossil''' is any [[fossil]]ized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.<ref name="Freeman">{{harvnb|Freeman|Herron|2004|p=816}}</ref> This is especially important where the descendant group is sharply differentiated by [[gross anatomy]] and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are [[human]] constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.<ref name=Prothero/>
In 1859, when [[Charles Darwin]]'s ''[[On the Origin of Species]]'' was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory," but he explained it by relating it to the extreme imperfection of the geological record.<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=297 279–280]}}</ref> He noted the limited collections available at the time but described the available information as showing patterns that followed from his theory of [[evolution|descent with modification]] through [[natural selection]].<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=359 341–343]}}</ref> Indeed, ''[[Archaeopteryx]]'' was discovered just two years later, in 1861, and represents a classic transitional form between earlier, non-avian [[dinosaur]]s and [[bird]]s. [[List of transitional fossils|Many more transitional fossils]] have been discovered since then, and there is now abundant evidence of how all [[class (biology)|classes]] of [[vertebrate]]s are related, including many transitional fossils.<ref name="NS2645">{{cite journal |last=Prothero |first=Donald R. |author-link=Donald Prothero |date=1 March 2008 |title=Evolution: What missing link? |url=https://www.newscientist.com/article/mg19726451.700-evolution-what-missing-link.html?full=true |journal=[[New Scientist]] |issue=2645 |pages=35–41 |issn=0262-4079 |doi=10.1016/s0262-4079(08)60548-5 |volume=197|url-access=subscription }}</ref> Specific examples of class-level transitions are: [[Evolution of tetrapods|tetrapod]]s and [[Evolution of fish|fish]], [[Origin of birds|birds and dinosaurs]], and [[Evolution of mammals|mammals and "mammal-like reptiles"]].
The term "missing link" has been used extensively in popular writings on [[human evolution]] to refer to a perceived gap in the [[Hominidae|hominid]] evolutionary record. It is most commonly used to refer to any new transitional fossil finds. Scientists, however, do not use the term, as it refers to a pre-evolutionary view of nature.
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