What is a unique derived trait?

A unique derived trait, also known as an apomorphy, is a characteristic that has evolved in a particular lineage but is not found in the ancestral species from which that lineage descends. This trait distinguishes members of a specific group from other related groups. For example, feathers in birds are a unique derived trait that differentiates them from other reptiles. Such traits are significant in the study of evolutionary Biology as they help in understanding the relationships and divergence between species.