opaque medium[ō′pāk ‚mēd·ē·əm]
A medium impervious to rays of light, that is, not transparent to the human eye.
A medium which does not transmit electromagnetic radiation of a specified type, such as that in the infrared, x-ray, ultraviolet, and microwave regions.
A medium which prevents the passage of particles of a specified type.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.