What the philosophers and psychologists have said is irrelevant because idealism is a fluid concept, to me and it rises up and dies down given each new generation’s concerns and issues. Besides, each of us (in my opinion, anyway) can only define in terms of how we think of our own idealism (assuming we have any) so (as with all your questions) I will define it as it pertains to me: an idealist is someone who is passionately involved in dreaming up/making happen a kind of reality that doesn’t (and may never) exist because that reality seems, to them, to be an answer to some of the ailments they observe in society.