How long it takes a customer to 'pay back' what you spent to acquire them.
Payback = CAC ÷ monthly gross profit per customer. Tells you how many months of customer payments you need before you recover the cost of acquiring them. Shorter = better. Under 12 months is healthy SaaS. Under 6 is elite. Over 18 is a warning sign, because it means you're fronting a lot of cash per new customer.
Payback period is how you manage the cash impact of growth. Even if LTV:CAC is great, a long payback means you need a LOT of cash to grow (because you're always ahead on acquisition costs). Shorten payback by raising price, lowering CAC, or moving to annual billing.