Metaphysics: a branch of philosophy concerned about the fundamentals of being and the world.

Universals: what particular things have in common, such as characteristics or qualities. They are also repetitive and recurrent entities that can be exemplified by many particular things. For example, suppose there are two chairs in a room, each of which is green. These two chairs both share the quality of "chairness," as well as greenness or the quality of being green. Metaphysicians call this quality that they share a "universal." There are three major kinds of qualities or characteristics: types or kinds (e.g. mammal), properties (e.g. short, strong), and relations (e.g. father of, next to). These are all different types of universal.

Nominalism: a metaphysical view in philosophy to which general or abstract terms and predicates exist, while universals or abstract objects, which are sometimes thought to correspond to these terms, do not exist. Thus, there are at least two main versions of nominalism. One version denies the existence of universals—things that can be exemplified by many particular things (e.g. strength, humanity). The other version specifically denies the existence of abstract objects—objects that do not exist in space and time.

Most nominalists have held that only physical particulars in space and time are real, and that universals exist only post res, that is, subsequent to particular things. Some versions of nominalism hold that some particulars are abstract entities (e.g. numbers), while others are concrete entities—entities that do exist in space and time (e.g. tables, chairs).

Nominalism is primarily a position on the problem of universals, which dates back at least to Plato, and is opposed to realism—the view that universals do exist over and above particulars. However, the name "nominalism" emerged out of debates in medieval philosophy with Roscellinus. As John Stuart Mill once wrote, the early versions of nominalism were that "there is nothing general except names", hence the prefix "nomin-". This, however, is a more dated use of the term that is now considered to be a specific version of what is now called "nominalism".