Full Idea
The haecceitist (a neologism coined by Duns Scotus, pronounced 'hex-ee-it-ist', meaning literally 'thisness') believes that each thing has an individual essence, a set of properties which are essential to it.
Gist of Idea
A haecceity is a set of individual properties, essential to each thing
Source
Stephen Read (Thinking About Logic [1995], Ch.4)
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.101
A Reaction
This seems to be a difference of opinion over whether a haecceity is a set of essential properties, or a bare particular. The key point is that it is unique to each entity.