symplesiomorphy
This term is used exclusively within the specialized field of cladistics and evolutionary biology. It describes a trait that is shared by a group of organisms but is not useful for determining their specific evolutionary kinship because the trait is too ancient, having been inherited from a distant ancestor common to many other groups.
In a taxonomic context, it is critical to distinguish this from synapomorphy, which refers to shared derived characters that actually define a specific clade. While a synapomorphy helps scientists group species together, a symplesiomorphy can be misleading if used as the sole basis for grouping, as it only indicates that the species belong to a broader, more inclusive lineage.
Meanings
The possession of a shared ancestral character state by two or more taxa, which does not provide evidence for their close relationship because the trait was present in the common ancestor of a larger group.
The presence of five digits on the limbs of most mammals is an example of symplesiomorphy, as it is a primitive trait inherited from an even more distant ancestor.