The Collaborative International Dictionary of English v.0.48:

Sabbath \Sab"bath\, n. [OE. sabat, sabbat, F. sabbat, L.
   sabbatum, Gr. sa`bbaton, fr. Heb. shabb[=a]th, fr. sh[=a]bath
   to rest from labor. Cf. Sabbat.]
   1. A season or day of rest; one day in seven appointed for
      rest or worship, the observance of which was enjoined upon
      the Jews in the Decalogue, and has been continued by the
      Christian church with a transference of the day observed
      from the last to the first day of the week, which is
      called also Lord's Day.
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            Remember the sabbath day, to keep it holy. --Ex. xx.
                                                  8.
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   2. The seventh year, observed among the Israelites as one of
      rest and festival. --Lev. xxv. 4.
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   3. Fig.: A time of rest or repose; intermission of pain,
      effort, sorrow, or the like.
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            Peaceful sleep out the sabbath of the tomb. --Pope.
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   Sabbath breaker, one who violates the law of the Sabbath.


   Sabbath breaking, the violation of the law of the Sabbath.


   Sabbath-day's journey, a distance of about a mile, which,
      under Rabbinical law, the Jews were allowed to travel on
      the Sabbath.
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   Syn: Sabbath, Sunday.

   Usage: Sabbath is not strictly synonymous with Sunday.
          Sabbath denotes the institution; Sunday is the name of
          the first day of the week. The Sabbath of the Jews is
          on Saturday, and the Sabbath of most Christians on
          Sunday. In New England, the first day of the week has
          been called "the Sabbath," to mark it as holy time;
          Sunday is the word more commonly used, at present, in
          all parts of the United States, as it is in England.
          "So if we will be the children of our heavenly Father,
          we must be careful to keep the Christian Sabbath day,
          which is the Sunday." --Homilies.
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Easton's 1897 Bible Dictionary:

Lord's day
   only once, in Rev. 1:10, was in the early Christian ages used to
   denote the first day of the week, which commemorated the Lord's
   resurrection. There is every reason to conclude that John thus
   used the name. (See SABBATH.)


Bouvier's Law Dictionary, Revised 6th Ed (1856):

LORD'S DAY. The same as Sunday. (q.v.) Dies Dominicus non est juridicus.
Co. Litt. 135; Noy's Max. 2.