The percentage of website visitors who complete a desired action (purchase, sign-up, or download).
CVR = (Total Conversions / Total Sessions) × 100
Improving your conversion rate is the highest leverage activity in marketing because it improves the efficiency of every single traffic source simultaneously.
If you pay $1.00 per click and have a 1% CVR, your CPA is $100. If you improve your CVR to 2% (via better landing pages), your CPA drops to $50 instantly. You just doubled your ROAS without touching your ads.
Benchmarks depend on price point and traffic temperature: