haecceity (n)

Latin: “thisness”

From Wikipedia:

“Haecceity” (/hɛkˈsiːɪti, hiːk-/; from the Latin haecceitas, which translates as “thisness”) is a term from medieval scholastic philosophy, first coined by followers of Duns Scotus to denote a concept that he seems to have originated: the discrete qualities, properties or characteristics of a thing that make it a particular thing. Haecceity is a person’s or object’s thisness, the individualising difference between the concept “a man” and the concept “Socrates” (i.e., a specific person).

Not to be confused with “quiddity,” which is a thing’s whatness.