Mirrorsoft was a software publisher in the United Kingdom. Owned by Mirror Group Newspapers, it was founded as a publisher of educational software before moving into games. One offshoot of its printing roots was Fleet Street Publisher on several platforms. A PC version was developed by Rowan Software but was scrapped before release.
Mirrorsoft's most famous product was Tetris, the rights to which were acquired in a series of complicated licensing issues.
Many games were not released under the name "Mirrorsoft", but under publishing labels like Image Works or Mirror Image.
Mirrorsoft was bought by Acclaim Entertainment in 1992.
Source: Wikipedia, "Mirrorsoft", available under the CC-BY-SA License.