How many dollars of customer lifetime value you get per dollar of acquisition cost.
LTV:CAC = lifetime value ÷ customer acquisition cost. The single most important ratio in marketing. Under 1:1, you're losing money on every customer, unsustainable. 1:1 to 3:1 is survive mode. 3:1+ is healthy. 5:1+ you have a machine, pour fuel in. Above 8:1 you're probably under-investing in growth and should spend more.
This ratio tells you whether your business model works at all. Track it quarterly. If it's drifting down, either CAC is creeping up or retention is slipping. Both matter; both are fixable. Don't pretend the number doesn't exist.