expression, noun
impossible dream; unattainable dream
A set phrase meaning a dream that cannot come true. The ぬ is a classical negative ending, giving a slightly literary or emphatic feel.
彼は叶わぬ夢を追い続けた。
He kept chasing an impossible dream.
叶わぬ夢だとわかっていても、諦められない。
Even though I know it's an unattainable dream, I can't give up.
More modern and colloquial version using ない instead of classical ぬ. 叶わぬ夢 sounds more literary or poetic.
From 叶う (to come true) in its negative classical form 叶わぬ + 夢 (dream). The phrase is a fixed expression.