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.
      [1913 Webster]

            Remember the sabbath day, to keep it holy. --Ex. xx.
                                                  8.
      [1913 Webster]

   2. The seventh year, observed among the Israelites as one of
      rest and festival. --Lev. xxv. 4.
      [1913 Webster]

   3. Fig.: A time of rest or repose; intermission of pain,
      effort, sorrow, or the like.
      [1913 Webster]

            Peaceful sleep out the sabbath of the tomb. --Pope.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

Sabbath
    n 1: a day of rest and worship: Sunday for most Christians;
         Saturday for the Jews and a few Christians; Friday for
         Muslims

The Devil's Dictionary (1881-1906):

SABBATH, n.  A weekly festival having its origin in the fact that God
made the world in six days and was arrested on the seventh.  Among the
Jews observance of the day was enforced by a Commandment of which this
is the Christian version:  "Remember the seventh day to make thy
neighbor keep it wholly."  To the Creator it seemed fit and expedient
that the Sabbath should be the last day of the week, but the Early
Fathers of the Church held other views.  So great is the sanctity of
the day that even where the Lord holds a doubtful and precarious
jurisdiction over those who go down to (and down into) the sea it is
reverently recognized, as is manifest in the following deep-water
version of the Fourth Commandment:

    Six days shalt thou labor and do all thou art able,
    And on the seventh holystone the deck and scrape the cable.

    Decks are no longer holystoned, but the cable still supplies the
captain with opportunity to attest a pious respect for the divine
ordinance.