A constraint is a limitation of possibilities. In particular, a constraint is a restriction of the feasible solutions in an optimization problem.
To take a simple example: a chain has 5 links, each link capable of holding a maximum weight of 5, 7, 4, 8 and 6 tonnes respectively. The maximum weight the chain can hold is clearly 4 tonnes – the limit imposed by the weakest link. In this case, all 5 links have limits to their strength, but the 3rd link is the constraint because it is the greatest restriction on the system.
Eliyahu M. Goldratt maintains that any business system has
- at least one constraint, otherwise its performance would be infinite
- very few constraints, otherwise it would be unstable and cease to exist
For constraints in literature, see constrained writing and in theatre, see Theatrical constraints.