Conversion rate
Conversion rate is the share of visitors who take the action a page exists for — a purchase, a form, a call — expressed as a percentage. It is the multiplier that decides what every visit, and every ad dollar, is actually worth.
Traffic and conversion multiply; they don't add. Doubling either doubles results, but conversion improvements are usually cheaper, because they compound across every channel at once: the same fix upgrades organic, paid, and referral visits together.
The definition needs discipline to be useful. “Conversion” must be an action with business value, not a proxy like time-on-page, and the rate must be read per intent: a pricing page and a blog post converting at the same rate would itself be a red flag.
The classic failure we build against: buying more traffic for a site that loses it. When leads grow slower than visits, the leak is conversion, and fixing the page beats raising the budget.