The Collaborative International Dictionary of English v.0.48:

Kindergarten \Kin"der*gar`ten\, n. [G., lit., children's garden;
   kinder (pl. of kind child, akin to E. kin kindred) + garten
   garden.]
   1. A class within a primary school or a separate school for
      young children, usually between the ages of four and six
      years, designed to adapt children to the classroom
      environment before beginning academic training, on the
      theory that education should be begun by gratifying and
      cultivating the normal aptitude for exercise, play,
      observation, imitation, and construction; -- a name given
      by Friedrich Froebel, a German educator, who introduced
      this method of training, in rooms opening on a garden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

kindergarten
    n 1: a preschool for children age 4 to 6 to prepare them for
         primary school