Not thought of before — and, in most of the sentences you will meet, better or bigger than anyone had thought of before. It is a written, slightly grand word, and it almost always sits in front of a noun about scale or success: unimagined riches, unimagined freedom, unimagined possibilities. The feeling is that the world went past what people had allowed themselves to hope for. There is a plainer use too, meaning simply "nobody pictured this", which can be neutral or even dark: "the unimagined cost of the war". But if a student meets this word in a novel or a speech, it is usually opening a door onto something wonderful. Note the difference from unimaginable: unimaginable is about what the mind CAN do — this thing is beyond the mind's power. Unimagined is about what the mind DID do — nobody happened to think of it, though they could have.

前所未想的；始料未及的。若後面接的是好事（unimagined riches），中文更自然的是「做夢也想不到的」。