The answer was given in music lingo above but I don't think it's adequate. Better explanations have been given elsewhere, such as on the session website http://www.thesession.org/ in fact here http://www.thesession.org/discussions/display/16653 , which is why people are sometimes asked to search archives. Why should we struggle to give a new definition here when we know that there's a perfectly good definition elsewhere that can be found with a quick search? That's not to say we should be rude about saying so, rather that we shouldn't be abused for bothering to answer the question in the first place.

I hope the above link answers your question.