The term medium has several different meanings in art. For starters, it is used to describe the substance that binds the pigment in paint. It is also called a binder, vehicle, or base. In acrylic paints, this is a synthetic substance. In oil paints, it is a natural oil such as poppy or linseed oil. In tempera paint, it is egg yolks. The plural is mediums.
To add to the confusion, the term medium is also used to describe the materials an artist uses, or simply the type of paint, such as oil, acrylics, or watercolors, as well the type of support, or surface used for painting. The plural is media. For example, the description "oil on canvas" indicates an oil painting done on a canvas surface; both oil and canvas are the media used. A "mixed-media" piece is one consisting of multiple media - for example, acrylic paint, graphite, and oil stick on a single surface or on a support consisting of canvas and newspaper.
Oil and acrylic paints can both be used straight from the tube and are very stable that way, but there are many different mediums that can be added to them to alter the drying time, finish, consistency and flow of the paint.
Mediums for oil painting include diluents and thinners such as turpentine and mineral spirits, as well as oils such as linseed oil, poppy oil, safflower oil, glaze medium, beeswax medium, and alkyd, which is a translucent synthetic resin that speeds up the drying time and increases paint flexibility. You should use as little medium as possible to get the effect you want with oil paints since they can cause some slight yellowing of your painting over time.
Acrylics are water-soluble and can be mixed solely with water for painting, or with a variety of acrylic mediums. There are many acrylic mediums available for different effects, from thinning and glazing to thickening and impasto effects; for a range of finishes from matte to glossy; for a textured finish; for retarding drying time; and for varnishing. Acrylic mediums appear white but become transparent when dry. Acrylic mediums (versus additives such as retarder) are made from the same acrylic polymer that acrylic paints are made from, so you can add as much as you want to the paint without affecting the chemical composition. With additives, you need to follow the directions on the tube or bottle.
Being familiar with mediums will help you follow the three cardinal rules in oil painting that will help prevent your paintings from cracking: