A Medium

1. (plural media or mediums) The chemistry of the surrounding environment, e.g. solid, liquid, gas, vacuum, or a specific substance such as a solvent.
2. (plural media or mediums) The materials or empty space through which signals, waves or forces pass.
3. (plural media or mediums) A format for communicating or presenting information.
4. (plural media or mediums, engineering) The materials used to finish a workpiece using a mass finishing or abrasive blasting process.
5. (plural media or mediums, microbiology) A nutrient solution for the growth of cells in vitro.
6. (plural media or mediums) The means, channel, or agency by which an aim is achieved.
7. (plural mediums or media) A liquid base which carries pigment in paint.
8. (plural mediums or media, painting) A tool used for painting or drawing.
9. (plural mediums, spiritualism) Someone who supposedly conveys information from the spirit world.
10. (plural mediums) Anything having a measurement intermediate between extremes, such as a garment or container.
11. (plural mediums) A person whom garments or apparel of intermediate size fit.
12. (plural mediums, Ireland, dated, informal) A half-pint serving of Guinness (or other stout in some regions).
13. A middle place or degree.
14. (dated) An average; sometimes the mathematical mean.
15. (logic) The mean or middle term of a syllogism, that by which the extremes are brought into connection.

(microbiology, nutrient solution): differential medium

(person claiming to convey information from the spirit world): mediumistic, mediumism, mediumship

(middle place or degree): happy medium, strike a medium