Part of the purchase price paid after closing, contingent on the business meeting agreed targets.
In a sale
An earn-out bridges a gap between what a founder believes the business is worth and what a buyer will underwrite. Whether it is fair depends on two things: whether the founder stays in a role that influences the result, and whether the buyer can influence the metric. An earn-out the founder cannot control is better read as a price cut than as part of the price.
Where it comes up
The Go-To-Market Analysis (GTMA) builds the file a buyer would build on your business, before a buyer does.