The Encyclopedia of Ancient Christianity provides a brief overview of the evolution of the Latin term persona (plural personae), which Tertullian is famous for introducing into discussions on the threeness of God. It begins with the secular meaning, and then moves to Tertullian's use of the word:
In addition to the classical meanings of the term persona (role, person and individuality), Latin theology from the very beginning knew of a technical meaning. [...] Borrowing from secular exegesis that distinguished persona grammatically and aesthetically (dignity or personal character), and under the influence of the legal tradition, [Tertullian] applied a realist meaning to the term (persona = res). (3:153)
Res is the Latin word for "thing" or "entity," which while pointing us in the right direction, perhaps still feels insufficiently specific. We can turn to J. N. D. Kelly, who writes in Early Christian Doctrines:
The primary sense of persona was 'mask', from which the transition was easy to the actor who wore it and the character he played. In legal usage it could stand for the holder of the title to a property, but as employed by Tertullian it connoted the concrete presentation of an individual as such. (115, emphasis added)
So we might say then that the personae of Tertullian are "persons" in the sense of "individuals" or "entities." But Kelly warns us against anachronistically reading modern understandings of "person" onto Tertullian:
In neither case, it should be noted, was the idea of self-consciousness nowadays associated with 'person' and 'personal' at all prominent. (115)
