Medium paint is a substance that is added to paint to change one or more of its properties. In acrylic mediums, these mixtures can be incorporated into the pigment to lengthen its drying time, make it thicker, or change the texture. Mediums are bought in addition to paints.
Varnish is a clear transparent hard protective finish or film. Varnish has little or no color and has no added pigment as opposed to paint or wood stain which contains pigment.