The Collaborative International Dictionary of English v.0.48:

Appurtenance \Ap*pur"te*nance\, n. [OF. apurtenaunce,
   apartenance, F. appartenance, LL. appartenentia, from L.
   appertinere. See Appertain.]
   That which belongs to something else; an adjunct; an
   appendage; an accessory; something annexed to another thing
   more worthy; in common parlance and legal acceptation,
   something belonging to another thing as principal, and which
   passes as incident to it, as a right of way, or other
   easement to land; a right of common to pasture, an outhouse,
   barn, garden, or orchard, to a house or messuage. In a strict
   legal sense, land can never pass as an appurtenance to land.
   --Tomlins. --Bouvier. --Burrill.
   [1913 Webster]

         Globes . . . provided as appurtenances to astronomy.
                                                  --Bacon.
   [1913 Webster]

         The structure of the eye, and of its appurtenances.
                                                  --Reid.
   [1913 Webster]

WordNet (r) 3.0 (2006):

appurtenance
    n 1: equipment consisting of miscellaneous articles needed for a
         particular operation or sport etc. [syn: gear,
         paraphernalia, appurtenance]
    2: a supplementary component that improves capability [syn:
       accessory, appurtenance, supplement, add-on]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "appurtenance":
   accession, accessory, accident, accidental, accommodation,
   accompaniment, addenda, addendum, additament, addition, additive,
   additory, additum, adjunct, adjuvant, advantage, amenity, annex,
   annexation, appanage, appendage, appendant, appendix, appliance,
   appointment, appurtenant, aspect, attachment, augment,
   augmentation, authority, auxiliary, birthright, circumstance,
   claim, coda, collateral, complement, component, concomitant,
   conjugal right, constituent, contents, contingency, contingent,
   continuation, convenience, corollary, demand, detail, divine right,
   droit, due, element, equipment, extension, extra, extrapolation,
   facility, factor, faculty, feature, fixings, fixture, furnishings,
   furniture, happenstance, inalienable right, incidental, increase,
   increment, inessential, ingredient, integrant, interest, item,
   makings, mere chance, natural right, nonessential, not-self,
   offshoot, other, part, part and parcel, pendant, power,
   prerogative, prescription, presumptive right, pretense, pretension,
   proper claim, property right, reinforcement, right, secondary,
   side effect, side issue, specialty, subsidiary, superaddition,
   supplement, tailpiece, title, undergirding, unessential,
   vested interest, vested right