The capital Latin letter I is used in math to represent the identity matrix sometimes with subtext to represent the size of the matrix.
For example, the symbol appears in an expression like this:
$I_{2}=[10 01 ]$
In plain language this means that the expression $I_{2}$ is equal to the square identity matrix with a width and height of two. More generically the expression $I_{n}$ represents the identity matrix with a width and height of $n$.
Symbol  Format  Data 

I  Unicode 

I  TeX 

SVG 