(philosophy) the philosophical theory that ideas are the only reality WordNet 3.0 & Open English WordNet

Idealism in philosophy, also known as philosophical idealism or metaphysical idealism, is the set of metaphysical perspectives asserting that, most fundamentally, reality is equivalent to mind, spirit, or consciousness; that reality is entirely a mental construct; or that ideas are the highest form of reality or have the greatest claim to being considered "real". Wikipedia

An approach to philosophical enquiry, which asserts that direct and immediate knowledge can only be had of ideas or mental pictures. Wiktionary

An approach to philosophical enquiry. Wiktionary (translation)