The Collaborative International Dictionary of English v.0.48:

Lyceum \Ly*ce"um\, n.; pl. E. Lyceums, L. Lycea. [L. lyceum,
   Gr. ?, so named after the neighboring temple of ? ? Apollo
   the wolf slayer, prob. fr. ? belonging to a wolf, fr ? wolf.
   See Wolf.]
   1. A place of exercise with covered walks, in the suburbs of
      Athens, where Aristotle taught philosophy.
      [1913 Webster]

   2. A house or apartment appropriated to instruction by
      lectures or disquisitions.
      [1913 Webster]

   3. A higher school, in Europe, which prepares youths for the
      university.
      [1913 Webster]

   4. An association for debate and literary improvement.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lyceum
    n 1: a school for students intermediate between elementary
         school and college; usually grades 9 to 12 [syn: secondary
         school, lyceum, lycee, Gymnasium, middle school]
    2: a public hall for lectures and concerts

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "lyceum":
   Gymnasium, Latin school, Realgymnasium, Realschule, academy,
   amphitheater, arena, assembly hall, auditorium, chapel,
   concert hall, convention hall, dance hall, exhibition hall,
   gallery, grammar school, hall, high, high school,
   intermediate school, junior high, junior high school, lecture hall,
   meetinghouse, middle school, music hall, opera house, prep school,
   preparatory school, public school, secondary school, seminary,
   senior high, senior high school, stadium, theater