In cladistics or phylogenetics, an outgroup is a group of organisms that serve as a reference group when determining the evolutionary relationship among three or more monophyletic groups of organisms. The outgroup is used as a point of comparison for the ingroup—the set of organisms under study that specifically allows the phylogeny to be rooted. Because the polarity (direction) of character change can only be determined on a rooted phylogeny, the choice of outgroup is essential for understanding the evolution of traits along a phylogeny. The chosen outgroup is hypothesized to be closely related to the other groups but less closely related than any single one of the other groups is to each other. The evolutionary conclusion from these relationships is that the outgroup species has a common ancestor with the ingroup that is older than the common ancestor of the ingroup. An outgroup may be a sister group to the ingroup or may be more distantly related.

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