Prior art is any evidence that your invention is already known.
Prior art does not need to exist physically or be commercially available. It is enough that someone, somewhere, sometime previously has described or shown or made something that contains a use of technology that is very similar to your invention.
While looking for prior art, you should also look for competing art. These are ideas that may not be at all like yours but do the same job. It is important to study competing art for two reasons:
From "What Is Prior Art?" (n.d.). Retrieved from https://www.epo.org/learning-events/materials/inventors-handbook/novelty/prior-art.html