Full Idea
Incidental properties have an incomplete essence, and need to refer in their definitions to their subject, lying outside their own genus.
Gist of Idea
Properties have an incomplete essence, with definitions referring to their subject
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.93)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.93
A Reaction
These are 'incidental' properties, but it is a nice question whether properties have essences. Presumably they must have if they are universals, or platonic Forms. The notion of being 'strong' can be defined without specific examples.