The Collaborative International Dictionary of English v.0.48:

Lawn \Lawn\ (l[add]n), n. [OE. laund, launde, F. lande heath,
   moor; of Celtic origin; cf. W. llan an open, clear place,
   llawnt a smooth rising hill, lawn, Armor. lann or lan
   territory, country, lann a prickly plant, pl. lannou heath,
   moor.]
   1. An open space between woods. --Milton.
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            "Orchard lawns and bowery hollows."   --Tennyson.
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   2. Ground (generally in front of or around a house) covered
      with grass kept closely mown.
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   Lawn mower, a machine for clipping the short grass of
      lawns.

   Lawn tennis, a variety of the game of tennis, played in the
      open air, sometimes upon a lawn, instead of in a tennis
      court. See Tennis.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tennis \Ten"nis\, n. [OE. tennes, tenies, tenyse; of uncertain
   origin, perhaps fr. F. tenez hold or take it, fr. tenir to
   hold (see Tenable).]
   A play in which a ball is driven to and fro, or kept in
   motion by striking it with a racket or with the open hand.
   --Shak.
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         His easy bow, his good stories, his style of dancing
         and playing tennis, . . . were familiar to all London.
                                                  --Macaulay.
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   Court tennis, the old game of tennis as played within
      walled courts of peculiar construction; -- distinguished
      from lawn tennis.

   Lawn tennis. See under Lawn, n.

   Tennis court, a place or court for playing the game of
      tennis. --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lawn tennis
    n 1: a game played with rackets by two or four players who hit a
         ball back and forth over a net that divides the court [syn:
         tennis, lawn tennis]