A liquid or a gel used to support the growth of a plant, animal, or microbial cell culture. Also known as a culture medium, it contains all the necessary nutrients to sustain growth. An undefined medium, also known as a complex medium, contains a rich source of amino acids, nitrogen, vitamins, and trace elements needed for growth, such as yeast extract. It is non-selective in that all species of microorganisms if present would be capable of growth. A defined medium, also known as a synthetic medium, has a composition in which all the chemicals used are known. A minimal medium contains all the nutrients required for growth except for certain amino acids. They are used to select for or against recombinant DNA microorganisms. See selective medium.