Retention
Retention is the share of customers who buy from you again within a given window, and the work you do to make that happen.
Acquisition gets a first order. Retention is what turns that order into a business. A 30% repeat rate roughly doubles the value of every dollar you spend acquiring customers.
The highest-leverage retention work is unglamorous: a post-purchase email flow, a replenishment reminder timed to the product's actual use cycle, and a support experience that resolves problems in one reply.
Measure it as repeat purchase rate over 90 days, cohort by cohort, rather than as a single all-time number.