group theory
/ˈɡruːp ˌθɪəri/noun
1 Mathematics and Science. The branch of algebra concerned with the properties and applications of groups.
2 Sociology. Any of various theories concerned with the dynamics within or between social groups, or with the influence of social groups on people's thinking or behaviour.
Origin
Late 19th century. From group + theory, in sense 1 after German Gruppentheorie.