A constitution is a set of laws that a set of people have made and agreed upon for government—often as a written document—that enumerates and limits the powers and functions of a political entity. These rules together make up, i.e. constitute, what the entity is.
A constitution confers specific powers to an organisation or institutional entity, established upon the primary condition that it abides by the said constitution's limitations. An organisation is constitutional to the extent that it contains systemic mechanisms of power control for the protection of the interests and liberties of its members.
From a dispute-resolution perspective, the constitution only becomes involved in determining the outcome in the cases where the contracts involved have failed to arrive at a resolution, for example through incompleteness or ambiguity. This is because contracts between individuals or group entities are more specific than those that apply to larger encompassing groups and time-frames. The constitution is the largest-scale and most general contract that all members have agreed to.