"Mastered by Nimbus" near the hub of a Compact Disc means the glass production master for that pressing was engineered by the CD manufacturing division of the UK-headquartered classical label Nimbus Records. This manufacturer had facilities in the UK (Wales, with CDs pressed starting in August 1984), the USA (VA, UT, and CA), and Luxembourg.
In late 1992, the manufacturing division was sold and renamed to Nimbus U.S.A. In 1994 it was renamed Nimbus CD International. It then merged with UK manufacturer Damont Audio, then effectively merged with CD-ROM giant R.R. Donnelley & Sons. In 1998, it was sold to Carlton Communications (Technicolor), in turn bought in 2001 by Thomson Multimedia.
Meanwhile, the Nimbus label continued to operate independently, and Nimbus Technology & Engineering, the CD/DVD mastering hardware division, merged with Optical Disc Corporation to become ODC Nimbus, makers of Nimbus Mastering Systems equipment.
While the Guidelines allow for adding a credit (role: Mastered By, artist/company name: Nimbus), which results in a listing on this artist page, it is preferred to rather add it as a company (LCCN: Glass Mastered At, name: Nimbus), which results in a listing on the Nimbus label page. The latter should be used for references to "Nimbus" without the "Mastered By" anyway.