The triple constraint is a model that describes the three most significant restrictions on any project: scope, schedule and cost.
The triple constraint is sometimes referred to as the project management triangle or the iron triangle. In the typical triangular model, scope, schedule and cost are constraints that form the sides of the triangle, with quality as the central theme. (An alternative to the triangle, the project management diamond, adds quality as the fourth side of the model and changes the central theme to customer expectations.)
The three constraints are interdependent: None of them can be altered without affecting one or both of the others. For example, if the scope of a project is increased, it is likely to take longer and/or cost more. Likewise, an earlier deadline is almost certain to either require more money or a less ambitious scope.
The difficulty of satisfying expectations for all three constraints is sometimes expressed as pick two: the concept that in any set of three desired qualities, only two can be delivered. If, for example, clients want to keep the budget low, the product is likely to take longer or be of lower quality.
An alternative model, STR, defines project scope as the product of time and resources put into it.
See a video explanation of the triple constraint model: