Customer concentration is the share of your income that comes from your single biggest customer. Above thirty percent is generally treated as risky: if that customer leaves, the business does not have a bad quarter, it has an emergency.
It is easy to miss, because high concentration usually arrives alongside good news. A large account that keeps growing looks like success right up until it is the only account that matters.
Related numbers
Active customers is how many separate customers have paid you recently. Revenue per customer is the average each one pays. Rising revenue per customer alongside a falling customer count is concentration increasing, even when total revenue is flat.