Medium is the technique or material used by an artist to produce a work of art. It also refers to the substance that binds the pigment in paint. In acrylic paints, this is a synthetic substance. In oil paints, it's a natural oil such as poppy. In tempera, it's egg yolks. Also something used to change the consistency of the paint. For example, a gel medium is used to thicken paint for impasto.