Overall, in scientific contexts the word "medium" refers to a substance or material in which something exists or grows or through which something can move or otherwise travel. The word "medium" and its plural, "media," can have multiple applications in various areas of science, from the soils and other materials for plant growth (growing media) in agriculture to the material that is used to grow bacteria in a biology lab setting (culture media). The word may also appear in certain physics settings, such as an excitable medium or an active laser medium, or in astronomy, such as the interplanetary medium that exists alongside planets in the solar system and the interstellar medium that exists between stars in outer space.
In horticultural contexts, the term "growing medium" can be used to refer to things as simple as soil, or the soil blends that are used as ideal settings for seeds to germinate and plants to flourish. This medium is intended to support and provide nutrients to the plant in addition to facilitating root growth.
In biological contexts, culture media are used in experimental settings in order to grow samples of bacteria, or cultures, for observation and research. Agar is a commonly used culture medium thanks to its solidity and its resistance to degradation.