The Collaborative International Dictionary of English v.0.48:

Sunday \Sun"day\, n. [AS. sunnandaeg; sunne, gen. sunnan, the
   sun + daeg day; akin to D. zondag, G. sonntag; -- so called
   because this day was anciently dedicated to the sun, or to
   its worship. See Sun, and Day.]
   The first day of the week, -- consecrated among Christians to
   rest from secular employments, and to religious worship; the
   Christian Sabbath; the Lord's Day.
   [1913 Webster]

   Advent Sunday, Low Sunday, Passion Sunday, etc. See
      under Advent, Low, etc.
      [1913 Webster]

   Syn: See Sabbath.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sunday \Sun"day\, a.
   Belonging to the Christian Sabbath.
   [1913 Webster]

   Sunday letter. See Dominical letter, under Dominical.


   Sunday school. See under School.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Sunday
    n 1: first day of the week; observed as a day of rest and
         worship by most Christians [syn: Sunday, Lord's Day,
         Dominicus, Sun]
    2: United States evangelist (1862-1935) [syn: Sunday, Billy
       Sunday, William Ashley Sunday]