What is a study?

Recently a student asked me "What is a study?" North Light Dictionary of Art Terms defines a study as "a drawing or painting  of a section or of a whole composition, usually detailed more carefully than a sketch." Charles Sovak in his book Oil Painting, Develop your Natural Ability (recommended)  says, "remember, no matter how beautifully you paint an object, it remains a mere study until it is artfully incorporated into a composition."

Study of a box by Pam Coulter

I think the distinction between "study" and "finished composition" has been somewhat blurred. Consider the fact that Monet painted "Impression, Sunrise" very quickly, generating the name of a whole artistic movement: Impressionism. It was hardly a finished composition in the sense that the "Old Masters" thought of it, so it was a study. Plein air painters today often follow the impressionist lead in the matter, finishing a painting in one session and on site. A study can be so attractive that it rivals or surpasses a finished work. Because of it's freshness and immediacy, it has charm. 

But, ok, what is a composition? (Is it unfair to end with a question?)