Revenue generated per dollar of ad spend. The classic ad-accountability metric.
ROAS = revenue attributed to ads ÷ ad spend. If you spent $10k on Meta ads and attributed $30k of revenue to them, ROAS = 3. Paid-advertising teams live by this number. But revenue isn't profit, so ROAS can mislead. A 3x ROAS on a 20%-margin product means you're losing money.
ROAS is the standard unit for talking about ad performance. Know your target ROAS (usually 3-5x for mid-margin businesses) and optimize campaigns against it. But ALWAYS cross-check against CAC, LTV, and margin, ROAS alone can make a losing business look great.