What does the 'medium mean in association with visual arts?

A medium is what ever you are making your piece of art out of. Such as: paper for a painting, clay for a clay bowl, wall or ceiling for a mural, etc.
A medium in art is an object or item used to make the piece of art that you are making. For example, if I drew a picture of a house, with some crayons, a pencil, and markers, the medium for that piece of art would be the pencil, the crayons, and the markers.