Definition - CPM stands for cost per mille. This is the latin Mille which is one thousand so CPM effective means cost per thousand impressions

For example a website might divide and sell it's ad inventory in blocks priced with CPM.

Other terms like CPC and CPA.

CPM bidding on ad exchanges, most exchanges final bid auction is based on CPM.

What affects cpm prices? Geo, device, ad type and size

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