A constraint is a restriction on the degree of freedom you have in providing

a solution. Constraints are effectively global requirements, such as limited

development resources or a decision by senior management that restricts

the way you develop a system.

Constraints can be economic, political, technical, or environmental and

pertain to your project resources, schedule, target environment, or to the

system itself.

Three types of constraints exist

Natural - bound by the laws of nature

External - enforced by outside agents

Perceived - assumed to be undesirable, prohibited or impossible