"Mile" is a Latin word means thousand therefore CPM is also known as Cost-per-Thousand.

CPM | Cost per Mile | Cost per Impression

CPM is the amount agreed by the advertisement companies to pay for every 1,000 impressions (amount of users who see the ad), no matter if a click-through the ad or conversion was made.
Usually, CPM is used with Banner-Ads while CPC (Cost per Click) is usually used for text-link ads.