What's your favourite number?

 

I've never even heard of Yvengeny Zamyatin.  i is simply the square root of negative one, there isn't that much else to explain.  It shows up a lot in advanced math (starting around Algebra II) as the roots to equations like X^2+1=0.  i is used to define the imaginary numbers (defined as a number times i) and the complex numbers (defined as the sum of an imaginary number and a real number).  Imaginary and complex numbers seem useless in practical applications of math, but they actually come up surprisingly often in physics.