The word medium has a number of uses:

• The most common meaning of the word medium is an average or mean in a range of sizes or conditions. Usually, the term is used when there are only a few different sizes in the range, rather than many. For example, items such as drinks at a fast food restaurant might come in three sizes: small, medium, and large.
• When relating to the bearing of something, a medium (bearer) (plural—media) is a process or object that can carry, transmit, or store something.
• A medium is an individual who claims the ability to receive or channel messages from spirits, ghosts, or other discorporate entities.
• In microbiology and other scientific disciplines, a medium is a nutrient system for the artificial cultivation of cells or organisms and especially bacteria.
• In communications, medium is an intervening substance through which a message or information is transmitted or carried on.
• In art, medium is a specific kind of artistic technique or means of expression or the materials used in a specific artistic technique. (also see Multimedia)
• A medium (optics) is something that is homogeneous on lengthscales comparable with the wavelength of the light being considered.
• Medium is the title of a 2005 American television series. See Medium (TV series).
• MEDIUM is an Architectural design company based in Montreal Canada.

The word medium comes directly from Latin, in which it means, "the one in the middle."

The plural, "mediums" is sometimes used. The plural, "media" is not to be confused with the technical term, "media", which pluralizes to "medias". Media, the technical term, is a plural noun, singular in construction, coming from the plural of "medium".

Interestingly, the peculiarities of the English language and its use of the same word for different meanings allow English speakers to make such statements as, "Small medium at large!" that are both comprehensible and mildly amusing.