CPM (cost per mille) is the price of one thousand ad impressions, calculated as spend divided by impressions, times 1,000. It is the currency of awareness and reach buying, and on the auction it reflects how much competition there is for the audience you are trying to reach.
How CPM is calculated
CPM is (spend ÷ impressions) × 1,000. Spend $500 to show an ad 100,000 times and your CPM is $5. It measures the cost of being seen, not the cost of a response, so a low CPM is only good if those impressions reach the right people.
Why CPM rises
On Meta and LinkedIn you are bidding against everyone else who wants the same audience, so CPM climbs as an audience narrows, as competition increases, and as you push a fixed audience harder. A rising CPM on a scaling campaign is often an early sign of audience saturation. Read it next to frequency and CTR, never on its own.