Definition of mirroring

Immediately reproducing every update to a read-write database (the principal database) onto a read-only mirror of that database (the mirror database) residing on a separate instance of the database engine (the mirror server). In production environments, the mirror server is on another machine. The mirror database is created by restoring a full backup of the principal database (without recovery).

Replicating data on different computers or devices to provide backup or improved performance

In an Internet sense, ‘mirroring’ refers to replicating the contents of a Web site at another site to minimize load on the main server, or to speed up connectivity in a specific geographic region.