i is an imaginary number. Basically if you stick i on the end of a number that means it's imaginary. Unfortunately I forget how you get an imaginary number, or what they are used for; but Wikipedia tells me that "In mathematics, an imaginary number (or purely imaginary number) is a complex number whose squared value is a real number not greater than zero." It is even more unfortunate that I still do not have a clue what that actually means. (How do you square something and get a negative number?) I should ask λ.

Also this comment was not directed at me, but I got a kick out of that icon and wanted to jump in and comment on it.