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"