Great question, and one I was discussing with the director of the local arts council gallery last week. I've heard it defined lots of different ways and remember participating in rather long and heated discussions about it as far back as high school.
In my humble opinion, in the broadest sense possible, art is how humans expresses their experiences.
It could be through writing, singing, sculpting, painting, building, cooking, baking, talking, dancing--you name it.
And every act of creation--which I define as making something from nothing--is worth celebrating.
Truly, when you think about it, anytime someone sits down with a pencil and draws something that act is special. They are making something, a drawing, where none existed before. Very cool!
Humans have a need to sort/organize/rank things so once something is created then it is judged, categorized, and value determined.
All of that stuff is NOT art.
All of that stuff is humans trying to make sense of their experiences by labeling them and assigning value.
But art, well art is the act of humans expressing experiences.
And to appreciate art all the viewer needs to do is be quiet and have their own experience with it.
What the artist was thinking when they made the artwork is immaterial. Once the artwork is shared with the public the creator no longer has any say in how it is viewed. Indeed, each person seeing the artwork has a unique experience with it.
What we do with that experience is completely our individual choice. And some people may use it to inform their own piece of art. Fun!