The IEEE 610.12-1990 standard defines a "requirement" in three parts. What are they?
A requirement is (1) a condition or capability a user needs to solve a problem, (2) a condition or capability a system must possess to satisfy a contract or standard, or (3) a documented representation of (1) or (2).
The three-part definition is deliberately layered, because the word "requirement" gets used loosely to mean three different things:
- User-side need — "A condition or capability needed by a user to solve a problem or achieve an objective." This is the problem as the stakeholder experiences it ("I need to recover my password without calling support").
- System-side obligation — "A condition or capability that must be met or possessed by a system or system component to satisfy a contract, standard, specification, or other formally imposed document." This is the obligation placed on the thing you build, often because a contract or a standard (like PCI-DSS) demands it, whether or not any user ever asked.
- The documented artifact — "A documented representation of a condition or capability as in (1) or (2)." A requirement is not real engineering until it is written down; the sentence in the specification is itself "a requirement".
Why it matters: the same need can be stated high-level or in fine detail, and a requirement may serve as the basis for a contract you bid for or be the basis of the contract itself — at which point its precise wording becomes legally binding. Pinning down which of the three meanings is in play avoids the classic confusion where one person means "the customer's wish" and another means "the line item in the specification we owe them".
Tip: Meanings (1) and (2) explain most requirement fights. A user need with no system obligation behind it is a wish; a system obligation with no user need behind it is usually a standard or a contract, and arguing that "nobody asked for it" will not make it go away.
Go deeper:
IEEE 610.12-1990 — Standard Glossary of Software Engineering Terminology — the standard itself, which fixed this three-part definition.- Requirement (Wikipedia) — how the same three parts propagated into BABOK and the later standards.