What is cool?

Cool is an aesthetic of attitude, behavior, comportment, appearance, style and Zeitgeist. Because of the varied and changing connotations of cool, as well its subjective nature, the word has no single meaning. It has associations of composure and self-control and often is used as an expression of admiration or approval.
There is no single concept of cool. One of the essential characteristics of cool is its mutability—what is considered cool changes over time and varies among cultures and generations.
Nick Southgate writes that, although some notions of cool can be traced back to Aristotle, whose notion of cool is to be found in his ethical writings, most particularly the Nicomachean Ethics, it is not confined to one particular ethnic group or gender.
The sum and substance of cool is a self-conscious aplomb in overall behavior, which entails a set of specific behavioral characteristics that is firmly anchored in symbology, a set of discernible bodily movements, postures, facial expressions and voice modulations that are acquired and take on strategic social value within the peer context.
Cool was once an attitude fostered by rebels and underdogs, such as slaves, prisoners, bikers and political dissents, etc., for whom open rebellion invited punishment, so it hid its defiance behind a wall of ironic detachment, distancing itself from the source of authority rather than directly confronting it.
Cool is also an attitude widely adopted by artists and intellectuals, who thereby aided its infiltration into popular culture. Sought by product marketing firms, idealized by teenagers, a shield against racial oppression or political persecution and source of constant cultural innovation, cool has become a global phenomenon that has spread to every corner of the earth. According to Dick Pountain and David Robins, concepts of cool have existed for centuries in several cultures.Dick Pountain and David Robins, Anatomy of an Attitude, Reaktion Books Ltd., 2000.
Cool has been used to describe a general state of well-being, a transcendent, internal peace and serenity. It can also refer to an absence of conflict, a state of harmony and balance as in, "The land is cool," or as in a "cool [spiritual] heart." Such meanings, according to Thompson, are African in origin. Cool is related in this sense to both social control and transcendental balance.
While slang terms are usually comprised of short-lived coinages and figures of speech, cool is an especially ubiquitous slang word, most notably among young people. As well as being understood throughout the English-speaking world, the word has even entered the vocabulary of several languages other than English.
Cool can be used to describe composure and absence of excitement in a person, especially in times of stress, and can refer to something that is aesthetically appealing. It is also used to express agreement or assent. Cool is often used as a general positive epithet or interjection which has a range of related adjectival meanings. Among other things, it can mean calm, stoic, impressive, intriguing, or superlative.