Cost per 1,000 ad impressions. How much you pay for reach, regardless of clicks.
CPM = ad spend ÷ (impressions / 1,000). Pronounced 'CPM' (the M is Latin 'mille' for thousand). It's how much you pay for 1,000 people to see your ad. Different platforms have very different CPMs: Google $5-20, Facebook $5-15, TikTok $2-10, LinkedIn $20-60+. CPM matters for brand campaigns and for comparing platforms' raw cost.
CPM is the cost floor of any ad campaign. For direct-response campaigns, CPM is secondary, you care about downstream conversion. For brand campaigns, CPM is primary, you're buying attention, and cost-per-attention is the metric.