A definitive agreement is the binding contract that completes the sale, often called the purchase agreement, drafted by the buyer's lawyers against the letter of intent and negotiated by every legal team.
In a sale
It makes the deal binding and writes down what the letter left open: the statements the seller makes about the business, the disclosure schedules that list every exception to them, and the conditions to clear before funds move. Everything that departs from the agreement's statements is written down, because a disclosed issue is a priced issue and a surprise is a retrade. Your own attorney reads and negotiates it with you.
Where it comes up
The Go-To-Market Analysis (GTMA) builds the file a buyer would build on your business, before a buyer does.