What is a Constraint?
A constraint is a limiting factor the project must work within — a fixed deadline, a capped budget, a regulation, a resource ceiling, a mandated technology. Unlike assumptions (things believed true), constraints are imposed and non-negotiable without escalation; both live in the assumption log.
Constraints shape every plan: scheduling squeezes around the fixed date, design bends around the regulation. The professional move is surfacing them at initiation — the constraint discovered in month six was always there, just unrecorded.
Worked example
A stadium renovation logs its constraints on day one: completion before the season opener (immovable), $80M cap (board-imposed), night work only on match-adjacent weeks (city permit), and steel from certified national suppliers (funding condition). Every one of them costs schedule or money — and because they were named in week one, they were priced in week two instead of litigated in month ten.