Universal (metaphysics)
Universals (used as a noun) are either properties, relations, or types, but not classes. It is worth noting that all four items are generally considered abstract, nonphysical entities. They are at least so considered by Platonic realists; there are others who use the terminology of properties, relations, etc., but who do not wish to be realists. Part of the difficulty, indeed, of understanding this problem is understanding the complex and confusing relations between theory and language, and what the use of language does, or does not, imply.
Universals are contrasted with individuals. 'Universal' used as an adjective is contrasted with particular -- and sometimes with concrete, although this contrast may be confusing since Hegelian and neo-Hegelian (e.g. British idealist) philosophy speak of a 'metaphysics concerning what universals are supposed to be, or (alternatively characterized) whether they exist; this is called the problem of universals.
