medium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Something, such as an intermediate course of action, that occupies a position or represents a condition midway between extremes.
• noun An intervening substance through which something else is transmitted or carried on.
• noun An agency by which something is accomplished, conveyed, or transferred.
• noun A means of mass communication, such as newspapers, magazines, radio, or television.
• noun The group of journalists and others who constitute the communications industry and profession.
• noun Computers Any of various kinds of storage devices, such as hard drives or digital audiotape.
• noun A person thought to have the power to communicate with the spirits of the dead or with agents of another world or dimension.
• noun A surrounding environment in which something functions and thrives.
• noun The substance in which a specific organism lives and thrives.
• noun A culture medium.
• noun A specific kind of artistic technique or means of expression as determined by the materials used or the creative methods involved.
• noun The materials used in a specific artistic technique.
• noun A solvent with which paint is thinned to the proper consistency.
• noun Chemistry A filtering substance, such as filter paper.
• noun A size of paper, usually 18 by 23 inches or 17.5 by 22 inches.
• adjective Occurring or being between two degrees, amounts, or quantities; intermediate: synonym: average.

from The Century Dictionary.

• noun That which holds a middle place or position; that which comes or stands between the extremes in a series, as of things, principles, ideas, circumstances, etc.; a mean.
• noun Technically— In mathematics, a mean. See mean.
• noun In logic, the mean or middle term of a syllogism.
• noun A size of paper between demy and royal. American printing-medium is 19 × 24 inches; American writing-medium, 18 × 23 inches; English printing-medium, 18 × 28 inches; English writing-medium, 17½ × 22 inches; American double medium, 24 × 38 inches; and American medium and a half, 24 × 30 inches.
• noun Anything which serves or acts intermediately; something by means of which an action is performed or an effect produced; an intervening agency or instrumentality: as, the atmosphere is a medium of sound.
• noun Specifically— In painting, any liquid vehicle, as linseed-oil, poppy-oil, varnish, or water, with which dry pigments are ground, or with which pigments are mixed by the painter while at work, in order to give them greater fluidity.
• noun In acoustics, a ponderable elastic substance, as air or other gas, water, etc., which transmits the energy of the sounding body in waves of condensation and rarefaction to the ear.
• noun In heat and light, that which transmits the energy of the heated or luminous body to a distance in undulatory waves; the ether.
• noun In bacteriology, the nutritive substance, either a liquid or a solid, in which or upon which the various forms of microscopic life are grown for study. The liquid media employed are infusions of hay, extract of beer-yeast, and broth of various kinds of meat. The solid media most used are eggs, slices of potatoes and carrots, agar-agar, and especially gelatin and the gelatinized serum of the blood of oxen. After being thoroughly sterilized by heat, they are usually placed in test-tubes, and inoculated with the form that it is desired to study; the cultures may then be observed through the glass.
• noun A person through whom, or through whose agency, another acts; specifically, one who is supposed to be controlled in speech and action by the will of another person or a disembodied being, as in animal magnetism and spiritualism; an instrument for the manifestation of another personality.
• noun Something of mean or medium weight, size, etc.
• Middle; middling; mean: as, a man of medium size.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Having a middle position or degree; mean; intermediate; medial
• noun That which lies in the middle, or between other things; intervening body or quantity.
• noun Middle place or degree; mean.
• noun (Math.) See Mean.
• noun (Logic) The mean or middle term of a syllogism; that by which the extremes are brought into connection.
• noun A substance through which an effect is transmitted from one thing to another.
• noun rare An average.
• noun A trade name for printing and writing paper of certain sizes. See Paper.
• noun (Paint.) The liquid vehicle with which dry colors are ground and prepared for application.
• noun (Microbiology) A source of nutrients in which a microorganism is placed to permit its growth, cause it to produce substances, or observe its activity under defined conditions; also called culture medium or growth medium. The medium is usually a solution of nutrients in water, or a similar solution solidified with gelatin or agar.
• noun A means of transmission of news, advertising, or other messages from an information source to the public, also called a news medium, such as a newspaper or radio; used mostly in the plural form, i. e. news media or media. See 1st media{2}.
• noun a current medium of exchange, whether coin, bank notes, or government notes.
• noun (Physics) the ether.
• noun that which is used for effecting an exchange of commodities -- money or current representatives of money.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The nature of the surrounding environment, e.g. solid, liquid, gas, vacuum, or a specific substance such as a solvent.
• noun The material or empty space through which signals, waves or forces pass.
• noun A format for communicating or presenting information.
• noun engineering The materials used to finish a workpiece using a mass finishing or abrasive blasting process.
• noun microbiology A nutrient solution for the growth of cells in vitro.