(n.) the quality of something that makes it ‘this’, as opposed to anything else
This is one of those words that’s probably harder to define than it is to explain etymologically.
A term from seventeenth century scholastic philosophy (bear with me here...) haecceity is literally the quality of something that makes it this, as opposed to that—or, for that matter, anything else.
It’s a term that was invented by advocates of John Duns Scotus, the Scottish medieval philosopher who established a groundbreaking school of metaphysical and theological philosophy in the thirteenth and fourteenth centuries. (When Duns’ theories later fell out of fashion, those who steadfastly held onto them came to be mocked as ‘dunces’—a word we still use today.)
One of the major channels of thought in Scotist thinking concerned the concept of identity. With that in mind, these thinkers concocted the concept of haecceity—a term for whatever property or properties each individual thing is believed to hold that makes it precisely that individual thing. It is person’s haecceity that not only differentiates them from all other people, ultimately, but also helps draw a distinction between that person and the broader concept of person, or people.
The word haecceity derives from the haec, the feminine form of the Latin word for ‘this’, hic. It was coined on the basis of an even earlier philosophical term, quiddity, for the basic essence of something that literally makes it what it is—quid being the Latin for ‘what’.
So while quiddity is literally the ‘whatness’ of something, haecceity is its ‘thisness’.