For me, an emotion. A feeling.
Here's a thought;
What other species greets a fellow with. "Hi, how are you?"
There is a phrase in every language for this. So from the dawn of communication, or at least some early point, humans were aware of each others feelings and emotions. Not only that, but were interested in them, albeit it for empathetic or mercenary reasons.
So art is a way to express and share our emotions. How does a painting make you feel?
A picture, play, movie, song, or dancing all provoke some kind of emotion within us.
The artist is portraying their emotions via whatever media they use and sharing this with anyone who cares to partake.
So what emotion am I portraying with the above picture? Well, I am happy to leave that to your own interpretation :)