Agree with this, only problem being that would be maybe too easy to get. But... i actually is a famous number. Whereas "imaginary number" definitely is not. It's just a type of number. So I had no idea what you were asking for there after i didn't work.
Good point about it being a type of number, I did initially have it as i before realising anyone who typed 'infinity' would automatically get it, I've amended the question to make clear it is a type of number.