The Collaborative International Dictionary of English v.0.48:

Atheneum \Ath`e*ne"um\, Athenaeum \Ath`e*n[ae]"um\, n.; pl. E.
   Atheneums, L. Athen[ae]a. [L. Athenaeum, Gr. 'Aqhn`aion a
   temple of Minerva at Athens, fr. 'Aqhna^, contr. fr.
   'Aqhna`a, 'Aqhnai`a, in Homer 'Aqh`nh, 'Aqhnai`n, Athene
   (called Minerva by the Romans), the tutelary goddess of
   Athens.]
   1. (Gr. Antiq.) A temple of Athene, at Athens, in which
      scholars and poets were accustomed to read their works and
      instruct students.
      [1913 Webster]

   2. A school founded at Rome by Hadrian.
      [1913 Webster]

   3. A literary or scientific association or club.
      [1913 Webster]

   4. A building or an apartment where a library, periodicals,
      and newspapers are kept for use.
      [1913 Webster]

WordNet (r) 3.0 (2006):

athenaeum
    n 1: a literary or scientific association for the promotion of
         learning [syn: athenaeum, atheneum]
    2: a place where reading materials are available [syn:
       athenaeum, atheneum]