A constitution is a set of rules defining a system of government. It describes the institutional structure, the conditions under which power is held and exercised, and the procedures through which the rules themselves may be changed.
It sits above ordinary laws. It has provisions for coping with the need for changes.
Most constitutions are written or codified and place less reliance on accepted practices or "conventions". One exception is the United Kingdom where all constitutional rules are not in one written document but are found in Acts of Parliament, common law or judicial law and conventions.
Our Commonwealth Constitution is a single document. The Western Australian Constitution, on the other hand, is found in many places and not in one single document.