A millionaire is someone with at least a million pounds or dollars — very rich. People also stretch it: “He spends like a millionaire” can mean only that someone throws money around, not that they have counted a million in the bank. The word is common and a little admiring; it is not slang, and it is not stiff. Though actually, a million does not buy what it bought a hundred years ago, so the fairy-tale shine has faded a little — but the word still means “at least a million.” English borrowed the whole word from French millionnaire; it looks like million plus a tail, and that surface reading is fair.