constraint (project constraint)

A constraint, in project management, is any restriction that defines a project's limitations; the scope, for example, is the limit of what the project is expected to accomplish. 

The three most significant project constraints -- schedule, cost and scope -- are sometimes known as the triple constraint or the project management triangle. A project’s scope involves the specific goals, deliverables and tasks that define the boundaries of the project. The schedule (sometimes stated more broadly as time) specifies the timeline according to which those components will be delivered, including the final deadline for completion. Cost (sometimes stated more broadly as resources) involves the financial limitation of resources input to the project and also the overall limit for the total amount that can be spent.

Project constraints are also considered to be somewhat mutually exclusive. In the project management triangle, it is assumed that making a change to one constraint will affect one or both of the others. For example, increasing the scope of the project is likely to require more time and money. 

That reality is also expressed as the pick two principle, which maintains that for any given set of three desired qualities or expectations -- such as "good, fast and cheap" -- it is likely that only two can coexist: A given product might be delivered quickly and inexpensively, for example, but the quality will suffer.