The Collaborative International Dictionary of English v.0.48:

Playground \Play"ground`\, n.
   A piece of ground used for recreation; as, the playground of
   a school.
   [1913 Webster]

WordNet (r) 3.0 (2006):

playground
    n 1: an area where many people go for recreation [syn: resort
         area, playground, vacation spot]
    2: yard consisting of an outdoor area for children's play

The Free On-line Dictionary of Computing (19 January 2023):

Playground

   A visual language for children, developed for Apple's Vivarium
   Project.  OOPSLA 89 or 90?