It is now quite standard to use the word "constitution" in the sense of "a written document that describes the legal framework by which a country is governed". What I'd like to understand in some detail is how this use of the word (or its obvious cognate in other languages) arose.
The Commonwealth constitution of 1653 was called an "Instrument of Government". That of 1657 was the "Humble Petition and Advice". The Enlightenment-era Corsican constitution of 1755 was called "Cunsulta generale di Corti", and as far as I can gather from its French translation, seems to use the word "constitution" only once, in the preamble, with a fairly unclear meaning. The US Declaration of Independence uses the word in a similar unclear sense "[King George] has combined with others to subject us to a Jurisdiction foreign to our Constitution, and unacknowledged by our Laws". The Articles of Confederation in 1777 do not use the word "constitution".
The OED (headword "constitution", entry (7) in the second edition/1989) does not offer very convincing examples. They do quote this from 1750: "England is now the only monarchy in the world that can properly be said to have a constitution", but obviously this cannot refer to a written document. In fact, even the US Constitution isn't completely clear as to whether the word "constitution" refers to the document itself or to the legal framework it describes ("We [...] do ordain and establish this Constitution for the United States of America").
On the other hand, the word (or rather the Latin "constitutio") seems to have been used to designate important laws edicted by the Roman emperor, or the pope.
Can anyone shed some light on this?
The American Constitution itself is really considered the first of it's kind. There are many things that influenced the Constitution, and gave the Constitution it's name.
The founding of the American Republic - and the constitution - was strongly influenced by the Roman Republic. The framers studied classical history extensively and often looked at the "Constitutio" which where edicts, decrees and rescripts that governed the Roman Republic and later empire.The Roman constitution was not a single document, but rather a series of precedents and traditions that formed the structure in which the government operated. Later the Roman Emperor declared the Constitutio Antoniniana, which granted citizenship to freemen living in the Roman Republic.
I believe (and I am a American history student, not a roman history) that using the term to describe the core laws that was popularized by Livy's Ab Urbe condita which described the history of the Roman Republic. Later the term described edicts from the emperor, and the most important decrees by the Pope (Apostolic constitution) starting in the 1570s.
The idea of the constitution being a legal contract was influenced by Jean-Jacques Rousseau's Social Contract. The Magna Carta, which restricted the powers of government, and the "British Constitution" which like the roman Constitution was mainly tradition based were also influential, and led to the idea that authority could be granted by agreement rather then by princely authority.
The early colonies where created on the basis of charters that granted colonies under the authority of the government of England. In 1630, the settlers of Connecticut formed their government not based off of the external charter, but instead drafted the "Fundamental Orders." When the colonies declared independence, they chose likewise to replace the defunct charters with documents, which they called "Constitutions"
Sources -
Ab Urbe condita - http://www.forumromanum.org/literature/livius/trans1.html
The Social Contract: http://www.constitution.org/jjr/socon.htm
The Radicalism of the American Revolution: http://www.amazon.com/Radicalism-American-Revolution-Gordon-Wood/dp/0679736883
The Creaton of the American Republic: http://www.amazon.com/The-Creation-American-Republic-1776-1787/dp/0807847232/ref=pd_sim_b_2?ie=UTF8&refRID=0CZ9HPT323HSRGHGG1WG
Ancient Rome in America: http://shc.stanford.edu/news/research/ancient-rome-america
The Founders and the Classics