The Collaborative International Dictionary of English v.0.48:

sandlot \sand"lot\ (s[a^]nd"l[o^]t), n.
   a vacant lot, especially one where children play games.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

sandlot \sand"lot\, Sand-lot \Sand"-lot`\, a.
   1. Lit., of or pert. to a lot or piece of sandy ground, --
      hence, pert. to, or characteristic of, the policy or
      practices of the socialistic or communistic followers of
      the Irish agitator Denis Kearney, who delivered many of
      his speeches in the open sand lots about San Francisco;
      as, the

   sand-lot constitution of California, framed in 1879, under
      the influence of sand-lot agitation.

   2. of or pertaining to a sandlot; -- used especially in
      reference to informal games played by children; as,
      sandlot baseball.
      [PJC]

WordNet (r) 3.0 (2006):

sandlot
    n 1: a vacant lot used by city boys to play games