CPM is a 'traffic brokering' industry term.
CPM means cost per thousand (generally referring to impressions). Used mainly when buying banner and text ads, or email ads. It is a common confusion that CPM means 'cost per million' because of the 'm'. Actually the 'm' is from the Romain Numeral "M" which means 1000 (one thousand).
For more information and traffic brokering educational programs see Theme Zoom.