This is a sociological definition of art.  A connotative form of definition is used, which specifies the necessary and sufficient conditions for a piece of work to be considered art.  In logic, a necessary condition is one that must be satisfied for the statement to be true, and a sufficient condition is one that, if satisfied, guarantees that the statement will be true.

The Necessary Condition

Art is the alternative form of communicating non-life-essential information...  Art is all about information.  In fact, artistic works usually communicate multiple layers of information, be it visual, musical, text, etc.  This information is not "life-essential" because society uses other means of communication for vital information, such as the Constitution, criminal laws, textbooks, newspapers, and many more.  The society provides well established ways of presenting and disseminating this information, unlike artistic expressions, which are constantly on a quest for new alternative forms.

The Sufficient Condition

...that defines man and society.  The "man and society" clause here is indivisible and represents a complex notion encompassing human existence through time and history.  Without this condition, a lot of social phenomena could satisfy the above definition.  For example, an anecdote or a cartoon may satisfy the necessary condition, but neither is usually considered art from a strictly cultural standpoint.  Moreover, indeed, a piece of work becomes art when it is accepted by a sufficient audience to define (or depict, or portray, or describe, or, like in the case of abstract art, to be taken as a symbol of the times or mindset of a group of people) the human existence.

Could this seemingly simple definition be true after centuries of contentious debate about what art is?  The answer may be less complex than expected.  Very few would argue whether the Mona Lisa, "Anna Karenina," or Beethoven's Fifth Symphony should be considered an artistic work.  Even in the case of works of much lesser caliber than those, we seem to know intuitively whether a particular work should be considered art.  That means that the definition is programmed in our subconsciousness.  It also means that the notorious arguments are not actually about what art is, but whether or not a particular work or genre itself should be considered art.

Two primary characteristics of art open it up for wild interpretations: the fact that an alternative method or form is used to communicate with the audience and the fact that it attempts to define human existence.

The alternative form is closely tied in with style in art.  What is style, and what function does it play in art?  It is easier to explain using a more trivial example, such as fashion, which employs some of the artistic principles.  Shopping for clothes, a young lady with a keen sense of style unmistakably picks needed articles out of hundreds of pieces scattered throughout many stores.  She uses them to create different outfits that transmit the same set of subliminal messages, saying she is "young, sexy, available, in with the right crowd," etc.

In other words, besides providing food for our conscious consumption, like an image or a story plot, art employs the language of style that appeals to our subconsciousness.  These languages evolved over time and are dynamic entities, constantly changing along with society and in conjunction with individual artists finding their unique voices.

However, the most difficult, ungrateful, and controversial task that art selflessly undertakes is the attempt to define man and society.  Societies in general and groups within the societies have a need to separate and identify themselves, often taking an artistic genre or a group of genres as their symbols and even ideology.  It sometimes makes artistic works victims of antagonism and animosity among various layers of society based on age, income, philosophical, and political affiliation, among other criteria.

We have a strong inner desire to include other characteristics of art when we attempt to define it for ourselves, like its aesthetic and entertainment value; ability to evoke emotion; the human creativity involved in the process; high degree of skill and professionalism needed to create or perform artistic works; and, ultimately, talent itself that encompasses all of the above.

However, in the final analysis, all these qualities of art prove to be consequential.  They may be present in one form and not present in another, and even talent and creativity are just as needed in the development of new industrial and financial products as in the works of art.  The twentieth century, like no other period, showed the relative nature of aesthetic quality in art.  As for entertainment value, it very much depends on the way artworks are presented and used.  Many forms of entertainment, like documentary films, books about animals and travel, etc., may or may not employ various artistic elements.

Because so many media use some form of artistic principles, like in our example with fashion, it makes the boundaries of art quite amorphous.

When all the dust of history settles, time becomes the ultimate judge of art.  Time makes the final determination of what forms of art are left to define and represent each historical epoch.