Avatāravāda, i.e. the doctorine of incarnation.
No other religious group uses it as extensively as the as the Vaiṣṇavas. In Vaiṣṇavism almost everyone is an incarnation of either Viṣṇu or some other god, sent by Viṣṇu. And other than that it has a long history of appropriating rivalling religious figures as incarnations into its canon.