Conversion rate
Conversion rate is the percentage of clicks or sessions that complete the desired action. It links media quality to outcome quality: traffic volume belongs to media, conversion rate is shared between audience quality, landing experience, offer and measurement working correctly.
In practice
A falling conversion rate is a diagnostic fork: audience drift, landing page or tracking breakage, or an offer losing relevance. Check tracking first - it fails more often than strategy.
Segment conversion rate by audience and source before optimising; the blended number hides where the problem lives.