A measure of whether customers would recommend you, scored from the classic 'how likely are you to recommend us?' question by netting promoters against detractors. NPS is a health signal for word of mouth, the cheapest acquisition you will ever get, and the precondition for a referral programme. The mistake is treating the score as the insight: the number matters less than the follow-up, 'why did you give that score?'. Watch it around service recovery too - customers whose problem was fixed fast and generously often rate higher than those who never had a problem at all, which is why support quality shows up directly in NPS.
Benchmark. 50+ is good, 70+ is excellent. Below 30, investigate the product or experience before spending harder on growth. A strong NPS is the green light for a double-sided referral programme, which can generate 10-25% of new customers.