medium /mē″dē·əm/ pl. media [L, medius, middle] , a substance through which something moves or through which it acts. A contrast medium is a substance that has a density different from that of body tissues, permitting visual comparison of structures when used with imaging techniques. A culture medium is a substance that provides a nutritional environment for the growth of microorganisms or cells. A dispersion medium is the substance in which a colloid is dispersed. A refractory medium is the transparent tissues and fluid of the eye that refract light.
