delusion; wild idea; (wild) fancy; (ridiculous) fantasy
彼の話は妄想に過ぎないと思った。
I thought his story was nothing but a delusion.
妄想と現実の区別がつかなくなった。
He could no longer distinguish delusion from reality.
壮大な妄想を抱いて笑われた。
I was laughed at for having grandiose fantasies.