illusion

An illusion is a false impression or a mistaken idea that tricks your mind into believing something that isn't really true.

Think of it like this: have you ever looked at a drawing or a picture and thought it was real, only to find out later that it's just a fake? Or maybe you've been walking along and seen something moving out of the corner of your eye, but when you turn around, there's nothing there. That can be an illusion too. People often talk about illusions in relation to magic tricks or optical effects, like 3D movies where things seem real but aren't really happening.