Idealism is the theory in the philosophy of mind that holds that mind is the ultimate stuff of reality, and matter depends on mind for its existence. It is opposed to materialism, which holds that matter is the ultimate stuff of reality, and mind depends on matter for its existence; and dualism, which holds that mind and matter are both separate and independent existence, and neither is reducible to the other.
Adherents of idealism include George Berkeley and J. M. E. McTaggart.
Idealism is frequently associated with the idea that material reality is somehow unreal. Many idealists have indeed believed this — famously, McTaggart claimed that the notion of time was so self-contradictory and incoherent that it must be illusory. But not all idealists agree with labeling material reality as "unreal" — an alternative idealist view, is that while it does not possess the same ultimate reality, the independent self-existence, which mind possesses, it nonetheless is real for what it is.