Definition of: medium

(mē′dē·əm) noun plural ·di·ums (always for def 5) or me·di·a (-dē·ə)
1. An intermediate quality, degree, or condition; mean.
2. A surrounding or enveloping element; condition of life; environment.
3. Any substance, as air, through or in which something may move or an effect be produced: Air is a medium of sound.
4. An intermediate means or agency; instrument: Radio is an advertising medium.
5. A person believed to be in communication with or controlled by the personality of someone deceased.
6. In painting, a liquid which gives fluency to the pigment.
7. A mathematical mean.
8. Bacteriol. A substance sterilized by heat in which bacteria, viruses, and other micro–organisms are developed: also culture medium.
9. A size of paper, usually 18 x 23 inches, between demy and royal.
—circulating medium A money currency.
—adjective
1. Intermediate in quantity, quality, position, size, or degree; middle.
2. Mediocre. [<L, orig. neut. sing. of medius middle]