The Collaborative International Dictionary of English v.0.48:

Esoteric \Es`o*ter"ic\ ([e^]s`[-o]*t[e^]"[i^]k), a. [Gr.
   'eswteriko`s, fr. 'esw`teros inner, interior, comp. fr. 'e`sw
   in, within, fr. 'es, e'is, into, fr. 'en in. See In.]
   1. Designed for, and understood by, the specially initiated
      alone; not communicated, or not intelligible, to the
      general body of followers; private; interior; acroamatic;
      -- said of discussions of technical topics and of the
      private and more recondite instructions and doctrines of
      philosophers. Opposed to exoteric.
      [1913 Webster]

            Enough if every age produce two or three critics of
            this esoteric class, with here and there a reader to
            understand them.                      --De Quincey.
      [1913 Webster]

   2. Marked by secrecy or privacy; private; select;
      confidential; as, an esoteric purpose; an esoteric
      meeting.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Esoteric \Es`o*ter"ic\, n. (Philos.)
      (a) An esoteric doctrine or treatise; esoteric philosophy;
          esoterics.
      (b) One who believes, or is an initiate, in esoteric
          doctrines or rites.
          [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

esoteric
    adj 1: confined to and understandable by only an enlightened
           inner circle; "a compilation of esoteric philosophical
           theories" [ant: exoteric]

The Devil's Dictionary (1881-1906):

ESOTERIC, adj.  Very particularly abstruse and consummately occult.
The ancient philosophies were of two kinds, -- _exoteric_, those that
the philosophers themselves could partly understand, and _esoteric_,
those that nobody could understand.  It is the latter that have most
profoundly affected modern thought and found greatest acceptance in
our time.