The cost to acquire a genuinely new customer, calculated against new customers only rather than total orders. Blending returning-customer revenue into your CAC makes acquisition look cheaper than it is and hides a leaky top of funnel. At Quad Lock our first orders were profitable after blended nCAC, which is what let us reinvest aggressively without raising.
Benchmark. Aim for a first order that is at least contribution-neutral after blended nCAC. If you need repeat purchases to break even on acquisition, your runway is hostage to retention.