In the field of mathematics, j is a number that is closely related to i, the most famous imaginary number of all. Here's an example: i^2=-1, and j^2=1, however, j does not =1. In equations dealing with hypercomplex numbers and other highly complicated mathematical fields, j will sometimes be used to replace i for the sake of simplicity.
Next time some jackoff thinks he's hot shit and introduces Graham's number to your math class so that it looks like he's a real whiz, kick his phony ass with j (Number)! Who's the hot shit now, huh?