This thread is long overdue on ET. What I hope to start with here is a place where anytime someone reaches a new definition that they think they have grasped, that they check this thread to see if someone has already done a good job of the definition of the term, and if not, that the person spends the time to add to the list of definitions here. Alternatively, someone can debate the definitions. Ideally, this thread can turn into a FAQ and all that think they have a good defintion should add it to this thread. I will start with the most basic of definitions.