CPI

A unit of measurement of the number of typographic characters that fit on a one inch line of printed type. This unit is mainly used to specify font sizes that have the same width for each character (monospaced), however it may also be used to give an average for fonts with characters of varying widths.

"Please use a type that is 10 characters per inch."

(electronic media) A unit of measurement of the number of characters per inch ("linear packing density") when storing data on certain types of magnetic media or tape, used to measure storage density that the encoding medium possesses.