A medium is a surface supplied with nutrients that is used to grow microorganisms or cells. The medium must be sterile prior to inoculation so that only the organism of interest is grown. It must also contain all materials needed for growth.
Media can be either solid, also known as agar, or liquid, as is the case of a nutrient broth. Media can be classified as defined or complex based on what type of microorganism is being grown and its individual needs. Defined media is necessary for bacteria that have specific growth needs. Complex media is useful for many different types of microorganisms with general growth needs.