Outgroup

In cladistics, whenever three or more monophyletic groups of organisms are compared, and all but one of them are more closely related to each other than any single one of them is to the last, the latter group is known as the outgroup. The evolutionary conclusion from this is that the outgroup branched from the parent group before the other two groups branched from each other.

Some examples, with outgroup on the right:
 * Humans, chimpanzees &mdash; gorillas
 * Placental mammals, Marsupials &mdash; Monotremes
 * Tetrapoda, Actinopterygii &mdash; Elasmobranchii
 * Chordates, Echinoderms &mdash; Mollusks