Short definition
CRO is the practice of getting more visitors to take the action you want — call, fill a form, or buy — without having to spend more on traffic.
In depth
Conversion Rate Optimization is the systematic process of improving the percentage of website visitors who complete a goal. Instead of buying more traffic, you make the traffic you already have worth more. CRO combines analytics, heatmaps, session recordings, A/B testing, and qualitative research to find friction points: confusing forms, slow pages, unclear calls to action, missing trust signals, weak headlines. For a service business, doubling conversion from 2% to 4% on the same ad budget effectively halves the cost of every lead.
Example
An HVAC company moves the phone number from the footer to a sticky mobile bar and sees mobile calls jump 38% with no extra ad spend.