Metrics
CPM (cost per mille)
CPM is the cost per thousand impressions, the standard pricing unit for display, video and social inventory. It measures the price of delivery, not its quality or outcome - a low CPM on unviewable or irrelevant inventory is expensive at any price.
In practice
CPM comparisons only make sense within like-for-like inventory: format, placement quality, audience and market all move the number legitimately.
Use CPM to plan delivery volume and diagnose auction pressure, and outcome metrics to judge success.