1.4.0.1 Built-in variables

MATLAB has some built-in variables. Try,

pi
i
j

MATLAB represents $\pi$ as pi and $\sqrt{-1}$ as i and j. You could reassign the variable i or pi the value 2 (i=2) but I think that is a bad idea. If you do something like that and regret it, you can restore a built-in variable to its original value with the clear command, e.g., clear pi. In fact, you can clear all of your variables with clear with no argument.