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。