A Formal Theory of Substances, Qualities, and Universals, http://ontology.buffalo.edu/bfo/SQU.pdf

Hard to give a definition for. Intuitively a "natural kind" rather than a collection of any old things, which a class is able to be, formally. At the meta level, universals are defined as positives, are disjoint with their siblings, have single asserted parents.

Alan Ruttenberg