Enhanced CD in the context of CD-ROM drive

A CD-ROM (/ˌsiːdiːˈrɒm/, compact disc read-only memory) is a type of read-only memory consisting of a pre-pressed optical compact disc that contains data computers can read, but not write or erase. Some CDs, called enhanced CDs, hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or digital video) is only usable on a computer (such as ISO 9660 format PC CD-ROMs).

During the 1990s and early 2000s, CD-ROMs were popularly used to distribute software and data for computers and fifth generation video game consoles. DVDs as well as downloading started to replace CD-ROMs in these roles starting in the early 2000s, and the use of CD-ROMs for commercial software is now rare.

A CD single is a music single in the form of a compact disc (CD). Originally the CD single standard (as defined in the Red Book) was an 8 cm (3-inch) "mini CD" (CD3); later on the term referred to any single recorded onto a CD of any size, particularly the 12 cm (5-inch) "full-size" disc (CD5). From a technical viewpoint, a CD single is identical to any other audio CD. The format started gaining popularity in the early 1990s, but quickly declined in the early and mid 2000s, in favor of digital downloaded singles and CD albums.

Commercially released CD singles can vary in length from two songs (an A side and B side, in the tradition of 7-inch 45-rpm records) up to six songs like an EP, which would be marketed as a maxi single in some regions. Some contain multiple mixes of one or more songs (known as remixes), in the tradition of 12-inch vinyl singles, and in some cases, they may also contain a music video for the single itself (this is an enhanced CD) as well as occasionally a poster. Depending on the country of release, there may be limits on the number of songs and total length for sales to count in singles charts.