noun. A group of people excluded from or not belonging to one's own group, especially when viewed as subordinate or contemptibly different.

noun. A group of organisms not part of the group under consideration, used for comparison when analyzing phylogenetic relationships.

noun. In cladistics, all the taxa included in a study that do not belong to the ingroup that is of immediate interest.

noun. The group of people who do not belong to one's own social group.

"An outgroup is an organism considered not to be part of the group in question, although it is closely related to the group."

ingroup  

outcrop  outgrow  outgrown