1. [n]  polished surface that forms images by reflecting light
2. [n]  a faithful depiction or reflection; "the best mirror is an old friend"
3. [v]  reflect as if in a mirror; "The smallest pond at night mirrors the firmament above"

1. \Mir"ror\, n. [OE. mirour, F. miroir, OF. also mireor,
   fr. (assumed) LL. miratorium, fr. mirare to look at, L.
   mirari to wonder. See {Marvel}, and cf. {Miracle},
   {Mirador}.]
   1. A looking-glass or a speculum; any glass or polished
      substance that forms images by the reflection of rays of
      light.
            And in her hand she held a mirror bright, Wherein
            her face she often view[`e]d fair.    --Spenser.
   2. That which gives a true representation, or in which a true
      image may be seen; hence, a pattern; an exemplar.
            She is mirour of all courtesy.        --Chaucer.
            O goddess, heavenly bright, Mirror of grace and
            majesty divine.                       --Spenser.
   3. (Zo["o]l.) See {Speculum}.
   {Mirror carp} (Zo["o]l.), a domesticated variety of the carp,
      having only three or fur rows of very large scales side.
   {Mirror plate}.
      (a) A flat glass mirror without a frame.
      (b) Flat glass used for making mirrors.
   {Mirror writing}, a manner or form of backward writing,
      making manuscript resembling in slant and order of letters
      the reflection of ordinary writing in a mirror. The
      substitution of this manner of writing for the common
      manner is a symptom of some kinds of nervous disease.
   

2. \Mir"ror\, v. t. [imp. & p. p. {Mirrored}; p. pr. & vb.
   n. {Mirroring}.]
   To reflect, as in a mirror.
   

1. Writing duplicate data to more than one device (usually two hard disks), in order to protect against loss of data in the event of device failure. This technique may be implemented in either hardware (sharing a disk controller and cables) or in software. It is a common feature of RAID systems.

Several operating systems support software disk mirroring or disk-duplexing, e.g. Novell NetWare.

See also Redundant Array of Independent Disks.

Interestingly, when this technique is used with magnetic tape storage systems, it is usually called "twinning".

A less expensive alternative, which only limits the amount of data loss, is to make regular backups from a single disk to magnetic tape.

2. An archive site or web site which keeps a copy of some or all files at another site so as to make them more quickly available to local users and to reduce the load on the source site. Such mirroring is usually done for specific whole directories or files on a specific remote server as opposed to a cache or proxy server which keeps copies of everything that is requested via it.

For example, src.doc.ic.ac.uk is the main UK mirror for the GNU archive at gnu.org. There are also several mirrors of this dictionary.