Ah. I get it. Yes, well, it spread to PF, so...
The standard definition of Conceptual Art back in the day was: art in which the concept of an artwork was
, itself, the artwork. This was a subset of Modern Art, one weird branch of it.
One of the best known conceptual artists was a performance artist named Chris Burden. He had a famous piece called "Breathing Water" in which he came out in front of the audience, stuck his face in a bucket of water, and inhaled as much of it into his lungs as he could, thus demonstrating the concept of breathing water.
Anyway, this thing with the invisible art is not where mainstream art is. It's retro, like some kids are trying to go back and be hippies.