Pronounced hek-SEE-uh-tee: the particular “thisness” that makes an individual thing this one. Medieval philosophers used it for the quality that cannot be exhausted by a list of general properties. Two cafés can have the same chairs, menu and paint color while feeling entirely different; haecceity names the specificity the inventory misses. Merriam-Webster
Use with restraint. The word is more at home in the thought after dinner than in the reservation notes.