What is Constitutional Convention?

Legal Definition
A constitutional convention is an informal and uncodified procedural agreement that is followed by the institutions of a state. In some states, notably those Commonwealth of Nations states that follow the Westminster system and whose political systems derive from British constitutional law, most government functions are guided by constitutional convention rather than by a formal written constitution. In these states, actual distribution of power may be markedly different from those the formal constitutional documents describes. In particular, the formal constitution often confers wide discretionary powers on the head of state that, in practice, are used only on the advice of the head of government.

Some constitutional conventions operate separate from or alongside written constitutions, such as in Canada since the country was formed with the enactment of the Constitution Act, 1867. Others, notably the United Kingdom which lack a single overarching constitutional document, unwritten conventions are still of vital importance in understanding how the state functions. In most states, however, many old conventions have been replaced or superseded by laws (called codification).
-- Wikipedia
Legal Definition
A duly constituted assembly of delegates or representatives of the people of a state or nation for the purpose of framing, revising, or amending its constitution.
-- Black's Law Dictionary
Legal Definition
A convention of delegates met to form or amend a constitution.
-- Ballentine's Law Dictionary