A document a company sends to several vendors describing what it needs, so proposals and prices can be compared on the same basis.
A vague RFP produces incomparable bids: the cheapest quote is usually the one that understood the least, and the gap appears later as change requests.
We answer with assumptions and exclusions stated plainly, and we tell a client when their RFP is specified too loosely to price honestly — a short paid discovery costs less than a fixed bid built on guesses.