Ontological Dividuation

The principle by which an entity is really distinct from another in existence.

This gets at a classic distinction between virtual and real properties (such as in Thomism): that is, the difference between two things being conceptually distinct vs. really distinct. For example, triangularity and trilaterality are not really distinct in a triangle but they are conceptually distinct; whereas, a car is really distinct from a sidewalk. When we talk about ontological, as opposed to epistemic, distinctions; we are interested only in how two entities are truly distinct from each other in existence as opposed to how we divide it up on our minds to comprehend it.

This distinction in existence can refer to many aspects of it. It could be referring to a distinction in form (e.g., this siamese twin is not identical to their twin); in matter (e.g., Socrates and Plato are both human males, but they are not the same male because their form was received by different matter); in substance (e.g., this car is not a bike); in existence (e.g., the essence of this car is distinct from its existence); or in suppositum (e.g., the Son is not the Father in God).