An outgroup in a cladogram is a species or group of species that is used as a reference point for determining the evolutionary relationships among the species being studied, known as the ingroup. The outgroup is typically a taxon that is more distantly related to the ingroup species. By comparing the traits of the outgroup to those of the ingroup, researchers can infer which characteristics are ancestral and which are derived, helping to clarify the evolutionary history of the ingroup.
For example, in a cladogram examining birds and reptiles, a crocodile might serve as an outgroup. By looking at the similarities and differences between the crocodile and the birds and other reptiles, scientists can better understand how these groups are related and the evolutionary changes that have occurred over time.