Trinity.

"The triune God is one simple indivisible essence in an irreducible threefold personal modification. That is, God's unity is characterized by modes of being in each of which the entire divine essence subsists in a particular way; this simultaneous, eternal existence in these three modes is the one divine essence. Accordingly, the persons of the godhead are not distinguished from the divine essence realiter; there are not three eternals, or three incomprehensibles, or three uncreated, or three almighties, or three gods. This is not to reduce the persons back into some anterior unity (that is, this does not 'confound the persons'), but simply to state that the persons are inseparable from the essence, and the essence inseparable from its threefold personal modification. Pater et filius et spiritus sanctus unus deus est: the singular verb is telling."

- John Webster, "Trinity and Creation", International Journal of Systematic Theology, Jan. 2010, 8.