Quality: Feature or aspect of something that itself is not a thing.
This is a fundamental idea and term, and therefore difficult to define. Linguistically, a quality is what is represented by a one-place predicate, and in that role also known as a property.
Note that it is rather doubtful that there are in reality any qualities, in as much as what seem to be qualities to the human sensory apparatus or brain, or what does correspond to a one-place predicate, tend to be relations and more complex than they appear, in human experience or human language.
See also: Predicate, Property, Representing
Aristotle, Brentano, Carnap, Cartwright, Eyck & Thysse, Jespersen, Körner, Lyons, Strawson