The contribution profit a customer generates over a stated window, usually 12 months, not their total revenue. 'Lifetime' with no window is a forecast wearing a costume, so always say which window you mean. Revenue-based LTV flatters you; margin-based LTV tells the truth. LTV only means something next to CAC, which is why the LTV:CAC ratio is the number investors and acquirers actually look at.
Benchmark. On a 12-month contribution LTV, an LTV:CAC of 3:1 to 5:1 is strong. 2:1 to 3:1 works only with payback inside 6 months, and below 2:1 is red unless first orders are profitable. Above 5:1 usually means you are underinvesting in growth.