CPM stands for cost-per-thousand Impressions. In a CPM pricing model (as used by TargetSpot), you pay for every 1,000 Impressions your Campaign receives.

For example: A TargetSpot Campaign with a Bid Price of $7.50 that generates 2,000 Impressions will actually cost you $15.00.

An Impression is a single play of your Campaign's Ad Unit to a single, targeted Internet Radio listener. Each time your Ad Unit is delivered to a targeted listener it counts as one Impression.