So here I present a definition I came up with myself. By that I mean that after decades of interacting with the world and those in it, after thousands of books read and hundreds of art projects initiated, I have now, all by myself (irony alert) found a suitable definition.
Of course it is not perfect; no definition is. There will be edge cases and apparent violations of the general provisions. And it is not a definition for all times and all places. To the ancients, art might have been "deification through the creation of likenesses". Somewhere else it might instead have been a "ritualistic process that abstracts societal roles and processes". In the eighteenth century it might have been "an expression of individual genius". Or "the skilled expression of universal truths". In the gallery world it might instead be "whatever the curator decides."
I just made all those up. I am sure a dictionary could provide similar definitions and many more. But instead, I've boiled my definition down to four words. For us here in the twenty-first century in the Western world it is appropriate.
Art: Questioning by creating artefacts.
Art has to create something in the world. This differentiates art from philosophy, which questions through language. Though there are many people who create things, only those who are doing so to question (the world, society, themselves) are artists. In other words, the purpose matters. It should also be implicit in the active form utilised in this definition that the process of creation might be more important than the finished piece.
For the purposes of my definition an artefact does not have to be physical, but rather is simply the product of an active agent, e.g.. the artist. Otherwise sound art would be eliminated, and I wouldn't want to do that! And dance is an artefact, even if it is constrained temporally for the duration of the performance (but nothing lasts forever).
Sometimes the purpose of a work is not clear, and so it can be hard to tell if it might be excluded or included. That's not a criticism of the definition, merely a difficulty in application. (I do not wish to eliminate difficulty, only to provide a useful starting point.)
The definition manages to distinguish art from craft and design. The latter do not present a question but rather solutions. The design process is indeed questioning but, if at all successful, its result is not.
Apply this definition to typical cases and see if it works for you. Is Cameron's film "Avatar" art? Is that person down at the market who paints landscapes an artist? Are video games art? Lady Gaga's "Poker Face"? That oil painting by the clever dog?
Have fun. And look out for those seed streamers in the sky. They tickle so. And they have an annoying way of feeling like they are still on your skin, long after you've brushed them off.