The doctrine that the application of a general term to various objects indicates the existence of a mental entity that mediates the application WordNet 3.0 & Open English WordNet

In metaphysics, conceptualism is a theory that explains universality of particulars as conceptualized frameworks situated within the thinking mind. Wikipedia

A theory, intermediate between realism and nominalism, that the mind has the power of forming for itself general conceptions of individual or single objects; the doctrine that universals have an existence in the mind apart from any concrete embodiment. Wiktionary

Theory. Wiktionary (translation)