haecceity
The essence of a particular thing that gives it its unique particularity; those qualities that make an individual this specific individual and not some other.
countable, uncountable
He [Ignatius of Loyola] ſaith not this of ſome one Church, but of all; nor yet as of an accident proper to thoſe times of the Churches minority; but as of the Notes of every Churches Individuation or Hæcceity as they ſpeak. The Unity of the Church is characteriſed by One Altar, and One Biſhop with the Presbytery and Deacons.
Where it comes from
From Medieval Latin haecceitas (“thisness”) coined by Duns Scotus (from haec (“this, these”)) + -ity.
Adapted from the Wiktionary entry haecceity (CC BY-SA 4.0).