A living fossil is a species or lineage that appears to have persisted with little apparent change over a long fossil interval.
The phrase is informal rather than a precise scientific category. It usually refers to organisms that retain some ancestral features and have close fossil relatives that look similar. The coelacanth is a famous example because living species resemble ancient coelacanth fossils in several important respects.
However, a living fossil has not stopped evolving. Genetic change, ecological adaptation, and anatomical differences can accumulate even when the overall body plan remains recognizable. The term can therefore be misleading if it suggests that evolution has paused.
Horseshoe crabs, ginkgo trees, and the tuatara are also commonly described this way, although each has a complex evolutionary history. Scientists generally prefer specific descriptions of morphological conservatism when making formal comparisons.