It seems that we at M.SE have developed a rather large repertoire of definitions of the word "context". Looking at the questions in the close vote queue for "missing context", one will see many comments addressing this fault - and these will range from asking only about what they've attempted, to asking about the literal context (i.e. what textbook) the question comes from - and each of these comments implies some definition of context. Some users seem to put substantial effort into outlining an idea of what would be acceptable context for new users, while others seem perfectly content with leaving a fairly useless comment of "What have you tried?" to every such question.
Given that "missing context" is almost certainly the most common reason for closing questions, we ought to discuss the following:
What is context? What sorts of context should the author of a question consider including?
A canonical answer to this question may be used to cut down on the number of "What have you tried" comments (i.e. to instead direct new users to a better reference - since it must be frustrating to have a question on which you have no idea and have it closed with no mention of what "context" actually is) and to give new users a better idea of our expectations (without spending unnecessary effort explaining the same thing to all such users).