1 〔知覚の間違い〕an illusion; 〔幻覚，妄想〕a hallucination
have [be under] an illusion／be subject to hallucinations
It looked round because of an optical illusion.
I was momentarily under the illusion that I was in a foreign country.
2 〔思い違い〕a mistake; a mistaken impression
I mistook him for his brother.