CPM
Cost per mille
The cost of one thousand impressions, which is the price of reaching an audience rather than of any result.
CPM = (ad spend / impressions) x 1000
What makes it wrong.
The mechanism, not the warning.
A rising CPM with flat performance is often read as fatigue when it is competition. The distinction is visible: fatigue shows a falling click-through rate on stable delivery, competition shows a rising CPM on a stable click-through rate.
In detail.
CPM is the auction's own price signal. It moves with demand for the audience being bought, and it moves for reasons that have nothing to do with the advertiser: a competitor's launch, a seasonal peak, an election.
Because it is upstream of everything, a CPM change explains many performance changes that are otherwise blamed on creative or targeting.