CPI (Counts Per Inch) is the notation originaly used by Steelseries for mice.

Whereas DPI is a dot per inch, CPI is a keyword that indicates how many times cursor information is sent to the computer side per inch.

CPI is a term that is in line with the meaning of the "sensitivity" of a gaming mouse, but nowday it's used very little. This is DPI that is general used, but you can consider it the same thing.