medium, media

Use medium, not media, as a singular subject. Ensure that the verb agrees with the subject (that is, the medium is and the media are).

Use media to describe:

• Materials or substances, such as fiber optic cable or wire, through which data is transmitted.
• Materials on which data is recorded or stored, such as magnetic disks, CDs, or tapes.
• The mass-communications industry and its practitioners, such as publishing and broadcasting.
• Journalists as a group, whether they're published in print or on the web, or broadcast.

Don't use media as a shortened form of multimedia.

Examples
The media include online broadcasts as well as newspapers, magazines, radio, and television.
The media covers computer industry news.
The medium used for many large computer programs is the DVD-ROM.