A constitution is a system for governance that establishes the rules and principles of an organisation, or more loosely, a group of people.
In the case of nations, it defines the fundamental political principles, and establishes the structure, procedures, powers and duties, of a government. Most national constitutions also guarantee certain rights to the people.
National constitutions seek to regulate the relationship between institutions of the state, in a basic sense the relationship between the executive, legislature and the judiciary, but also the relationship of institutions within those branches. For example, executive branches can be divided into a head of government, government departments/ministries, executive agencies and a civil service/bureaucracy. Most constitutions also attempt to define the relationship between individuals and the state, and to establish the broad rights of individual citizens. It is thus the most basic law of a territory from which all the other laws and rules are hierarchically derived; in some territories it is in fact called "Basic Law."
Generally, all constitutions give specific powers to an organization on the condition that it abides by this constitution or charter limitation.