Source : Webster's Revised Unabridged Dictionary (1913)

Domain \Do*main"\, n. [F. domaine, OF. demaine, L. dominium,
   property, right of ownership, fr. dominus master, owner. See
   {Dame}, and cf {Demesne}, {Dungeon}.]
   1. Dominion; empire; authority.

   2. The territory over which dominion or authority is exerted;
      the possessions of a sovereign or commonwealth, or the
      like. Also used figuratively.

            The domain of authentic history.      --E. Everett.

            The domain over which the poetic spirit ranges. --J.
                                                  C. Shairp.

   3. Landed property; estate; especially, the land about the
      mansion house of a lord, and in his immediate occupancy;
      demesne. --Shenstone.

   4. (Law) Ownership of land; an estate or patrimony which one
      has in his own right; absolute proprietorship; paramount
      or sovereign ownership.

   {Public domain}, the territory belonging to a State or to the
      general government; public lands. [U.S.]

Source : WordNet®

public domain
     n : property rights that are held by the public at large

Source : Free On-Line Dictionary of Computing

public domain
     
        (PD) The total absence of {copyright} protection.  If
        something is "in the public domain" then anyone can copy it or
        use it in any way they wish.  The author has none of the
        exclusive rights which apply to a copyright work.
     
        The phrase "public domain" is often used incorrectly to refer
        to {freeware} or {shareware} (software which is copyrighted
        but is distributed without (advance) payment).  Public domain
        means no copyright -- no exclusive rights.  In fact the phrase
        "public domain" has no legal status at all in the UK.
     
        See also {archive site}, {careware}, {charityware},
        {copyleft}, {crippleware}, {guiltware}, {postcardware} and
        {-ware}.  Compare {payware}.