synapomorphy

This term is a highly specialized technical expression used almost exclusively within the fields of phylogenetics and evolutionary biology. It describes a specific type of trait that allows scientists to group organisms into a clade, distinguishing them from ancestors who lacked the trait.

It is frequently contrasted with symplesiomorphy, which refers to a shared ancestral trait that is not useful for defining a specific subgroup because it is too widespread. Using this word in a non-scientific context would be considered inappropriately jargon-heavy and likely incomprehensible to a general audience.