Thanks! Before your explanation I assumed marzenie encompassed all meanings of the English dream. Now I differentiate:
sen: what may happen during sleep.
marzenie: vision, prophecy, fantasy. MLK's speech would have worked almost as well with the more accurate vision (still rejected by Duo!), but flows better with the monosyllabic dream.
mrzonka (pejorative!): chimæra, crazy idea, pipe dream, phantasm
halucynacja: hallucination; waking dream
Does anyone know a Polish word for daydream without the negative tone of mrzonka?