coessentiality

Participation in the same essence.

[…] Coeſſentiality, Sameneſs of Nature, and Sameneſs of Eſſence, […] do all of them, in the ſenſe of the Fathers, denote but one and the ſame Thing, viz. A Numerical Unity of the Divine Nature, […]

The condition or quality of being coessential (“mutually essential for something else”).