The power of a people or founding body to create or remake a constitution, as opposed to the 'constituted' powers that act within it.
In more detail
Sieyès, in What Is the Third Estate? (1789), argued that the nation holds constituent power and that legislatures and governments are constituted bodies bound by the constitution. Later theorists point to a paradox: constituent power seems to come before law, yet it is exercised through legal forms such as assemblies and referendums, and it is unclear who 'the people' are before the constitution defines them.