Nominalism is the doctrine that abstract concepts, general terms or universals have no independent existence but exist only as names. Therefore, various objects labelled by the same term have nothing in common but their name. Put another way, only actual physical particulars are real, and universals exist only subsequent to particular things, being just verbal abstractions.

Nominalism arose in reaction to the problem of universals and in particular to Plato's solution to it, known as Platonic Realism, which holds that abstract objects like universals and Forms exist in their own right and are wholly independent of the physical world, and that particular physical objects merely exemplify or instantiate the universal. Nominalists ask exactly where this universal realm might be, and find it unusual and unlikely that there could be a single thing that exists in multiple places simultaneously.

There have been attempts to bridge the gap between Realism and Nominalism including Moderate Realism (the view that there is no separate realm where universals or universal concepts exist, but that they are located in space and time wherever they happen to be manifest) and Conceptualism (the doctrine that universals exist only within the mind and have no external or substantial reality).

The Medieval French Scholastic philosopher and theologian Roscellinus of Compiegne (c. 1050 - 1125), a teacher of Peter Abelard, is often regarded as the founder of modern Nominalism.

William of Ockham is also considered a pioneer of Nominalism, and he argued strongly that only individuals exist (rather than supra-individual universals, essences or forms), and that universals are the products of abstraction from individuals by the human mind and have no extra-mental existence. However, his view is perhaps more accurately described as Conceptualism rather than Nominalism, holding that universals are mental concepts (which do exist, even if only in the mind) rather than merely names (i.e. words rather than existing realities).

• Predicate Nominalism takes the linguistic line that, for example, two individual cats are both cats simply because the predicate "cat" applies to both of them (although to some extent this still begs the question of what the predicate actually applies to).
• Resemblance Nominalism holds that "cat" applies to both cats because they resemble an exemplar cat (an exemplar is a model or pattern to be copied or imitated) closely enough to be classed together with it as members of its kind, or that they differ less from each other (and other cats) than they differ from other things.
• Psychological Nominalism is the view in psychology that explains psychological concepts in terms of public language use.