A universal can be defined as an abstract object or term which ranges over particular things.
Realists argue that they do.
Plato, the first and most extreme realist, argued that universals are forms and exist in their own spiritual realm. Individual objects, such as a large mountain, then participate in the universal form largeness. A universal can only be known by the intellect, and not the senses. The particulars, for Plato, are only manifestations of the forms. Plato's theory is subject to the problem of explaining how universals are represented in their particulars and how a universal can reside in a particular.