We think of Plato as the first Idealist. Literally this is the view that ideas themselves are more real in their abstract nature than what we think of particular things. This view is dominated by the view that mathematics is a more certain way of knowing than empirical observation.
Berkeley is unique since he uses empiricism as the grounds for his epistemology yet develops an Idealism in order to return the focus on God and away from Material Substance.
Hegel’s Idealism brings an end to epistemology as a problem of a knower separated from reality that the knower is trying to know. Following Hegel we are engaged in postmetaphysical thinking or postmodernism.