1. The quality or property that makes something uniquely itself; the essence of an individual thing that distinguishes it from all others, even those identical in all other respects.
Usage: philosophy; formal; rare

Examples

• In medieval philosophy, haecceity was the concept used to explain what makes one person distinct from another person with identical properties.
• The philosopher argued that haecceity, not mere physical characteristics, determines the identity of an object.
• Some metaphysicians debate whether haecceity is a real property or merely a logical construct.
• The haecceity of a work of art lies in its unique historical and material existence, not just its appearance.
• Without the notion of haecceity, it would be difficult to explain why two identical twins are still two different individuals.