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]

Moby Thesaurus II by Grady Ward, 1.0:

155 Moby Thesaurus words for "esoteric":
   absolute, abstract, abstruse, anagogic, arcane, auricular,
   between the lines, between us, cabalic, cabalistic, censored,
   certain, classified, close, closed, concealed, concrete,
   confidential, covert, cryptic, dark, deep, deep-seated, defined,
   definite, delitescent, detailed, determinate, different, distinct,
   distinguished, dormant, eerie, enigmatic, especial, exceptional,
   express, extramundane, extraordinary, extraterrestrial, fey, fixed,
   heavy, hermetic, hibernating, hidden, hush-hush, hypernormal,
   hyperphysical, immanent, implanted, implicit, in petto,
   inalienable, individual, indwelling, infixed, ingrained, inherent,
   inner, inside, internal, intimate, intrinsic, inward, inwrought,
   irreducible, latent, lurking, metaphysic, minute, muffled,
   mysterious, mystic, not for publication, noteworthy, numinous,
   obfuscated, obscured, occult, off the record, otherworldly,
   particular, personal, possible, potential, precise, preterhuman,
   preternatural, preternormal, pretersensual, private, privileged,
   profound, psychic, recondite, resident, respective, restricted,
   sealed, secret, several, singular, sleeping, smothered,
   solipsistic, special, specific, spiritual, stifled, subjective,
   submerged, superhuman, supernatural, supernormal, superphysical,
   supersensible, supersensual, suppressed, supramundane,
   supranatural, theosophical, theosophist, top secret,
   transcendental, transmundane, ulterior, unalienable, unbreatheable,
   unchallengeable, under privilege, under security,
   under the surface, under wraps, underlying, undisclosable,
   undisclosed, undivulgable, undivulged, unearthly, unhuman,
   unmanifested, unpublishable, unquestionable, unrevealable,
   unrevealed, unspoken, untellable, untold, unutterable, unuttered,
   unwhisperable, unworldly, veiled, virtual



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.