When speaking of art, context is the overall criteria by which it is judged and defined in the 21st century. Now that sentence warrants a break down and explanation of its components.
So let's begin at the end of the sentence. I single out America for several reasons. Americans like to compartmentalize things, in this case "art" and "not art." Within the category of art, there are multiple sub-categories broken down by taste (high and low art), period (impressionist, modern, etc), media (painting, sculpture), style (fine, folk, graphic, etc), subject (still life, erotic and so forth) and so on and so on. Also as Americans we must make these categories competitive, we cannot simple enjoy them for the beauty and visceral pleasure they bring. So Renaissance marble sculpture has a higher cultural ranking then modern abstract painting and impressionist landscapes are accepted more easily then gallery installations. Finally as with all things, American society places the rankings according to the monetary value the work can generate. Going back to previous examples a Michelangelo is priceless, but a Pollack has a price and a Monet will definitely go up in value, but the latest star coming out of Chelsea may not.
I stress that this is an American concept because our society is still so young and puritanical. Europeans live everyday of their lives with great art to the point it is sickening. Indigenous people have no concept of "art" because what we would prize and put on a wall or in a Plexiglas box, they use in their everyday life and rituals. Other societies and cultures view and accept art on a much more emotional level, unlike Americans that must intellectualize it and define it.
Now another part of the 1st sentence was the speculation of time. To be accurate, defining art is not a 21st century problem, but it is recent. A hundred plus years ago the definition of art was much simpler. Then came Duchamp and that damned urinal and everything got turned upside down. Art transformed and broke away from accepted schools of thought and movement beget movement, Picasso welded a handle bar to a bicycle seat, Andy Warhol became fascinated with Campbell's soup, Pollock threw paint wherever he damn well pleased, Kiki Smith collected bodily fluids and Nevelson painted wood black. As technology grows so does art. Now there is video and installation art in galleries and museums. Art created by lasers and complex computer programs, art online, on youtube, myspace, facebook, flickr and twitter. Art everywhere! (Whether it is good is another subjective matter.)
Finally I come to the crux of the first sentence, context. Context feeds the aforementioned examples and then in turn feed the context. A painting hanging in The Met is undoubtedly art. A video playing in a dark room is art, but opinions will be varied on it's quality. A tea bowl in Vermont thrown by a local is art but only as long as it remains on display and tea never touches it. However, as I believe it to be in all things, the final word on the subject is yours. If you believe it to be art then no professor, curator, tour guide, critic or expert's opinion matters. If it brings you joy, then enjoy it, because it si art to you.