1 [C,U] a false idea, belief or impression 虚幻的观念;幻想;幻觉:
I have no illusions about the situation -- I know it's serious.我知道形势严峻,对它不存幻想。I think Peter's under the illusion that he will be the new director.我想彼得误以为自己是下一任的董事长。
2 [C] something that your eyes tell you is there or is true but in fact is not 错觉:
That line looks longer, but in fact they're the same length. It's an optical illusion.那条线看上去长一点,其实两条线都一样长,这是视错觉使然。