They are the physical parts of media you would get when you buy something in a record store. E.g. they are the individual discs, vinyls contained within the packaging of an 'album/release/...'. In MusicBrainz terminology, mediums are included in a release. They contain information about the format (audio disc, 12", cassette, etc.), and the position in the release (e.g. disc 1 and disc 2). They can have an optional title (e.g. disc 2: The Early Years).
Disc IDs are linked to media. The content of a medium (which tracks and in which order) is given by a tracklist that is linked to the medium. Tracklists also have a MusicBrainz-specific meaning. A single tracklist can be linked to multiple media.
These CDs are media (mediums?) of their respective releases.
- CD1 of the 1984 US release of "The Wall" by "Pink Floyd"
- CD2 of the 2005 UK release of "Aerial" by "Kate Bush", named "A Sky of Honey"
When adding a medium to an existing release, please ensure that year of release does not conflict with the medium being added. Here are the dates of first commercial releases for some common formats: 7" records: c.1895 10" records: c. 1898 12" records: 1903 Reel-to-Reel: 1954 8-Track: 1964 Cassettes: late 1965 Compact Disc: October 1, 1982 SACD: September 1999 DVD-A: 2000