Haecceity (; from the Latin haecceitas, which translates as "thisness") is a term from medieval scholastic philosophy, first coined by Duns Scotus, which denotes the discrete qualities, properties or characteristics of a thing which make it a particular thing. Haecceity is a person or object's "thisness", the individualising difference between the concept 'a man' and the concept 'Socrates' (a specific person). Haecceity is a literal translation of the equivalent term in Aristotle's Greek to ti esti (τὸ τί ἐστι) or "the what (it) is."

Haecceity (; from the Latin haecceitas, which translates as "thisness") is a term from medieval scholastic philosophy, first coined by Duns Scotus, which denotes the discrete qualities, properties or characteristics of a thing which make it a particular thing. Haecceity is a person or object's "thisness", the individualising difference between the concept 'a man' and the concept 'Socrates' (a specific person). Haecceity is a literal translation of the equivalent term in Aristotle's Greek to ti esti (τὸ τί ἐστι) or "the what (it) is."