The Collaborative International Dictionary of English v.0.48:

Convenience \Con*ven"ience\ (?; 106), Conveniency
\Con*ven"ien*cy\, n. [L. convenientia agreement, fitness. See
   Convenient.]
   1. The state or quality of being convenient; fitness or
      suitableness, as of place, time, etc.; propriety.
      [1913 Webster]

            Let's further think of this;
            Weigh what convenience both of time and means
            May fit us to our shape.              --Shak.
      [1913 Webster]

            With all brief and plain conveniency,
            Let me have judgment.                 --Shak.
      [1913 Webster]

   2. Freedom from discomfort, difficulty, or trouble;
      commodiousness; ease; accommodation.
      [1913 Webster]

            Thus necessity invented stools,
            Convenience next suggested elbow chairs. --Cowper.
      [1913 Webster]

            We are rather intent upon the end of God's glory
            than our own conveniency.             --Jer. Taylor.
      [1913 Webster]

   3. That which is convenient; that which promotes comfort or
      advantage; that which is suited to one's wants; an
      accommodation.
      [1913 Webster]

            A pair of spectacles and several other little
            conveniences.                         --Swift.
      [1913 Webster]

   4. A convenient or fit time; opportunity; as, to do something
      at one's convenience.
      [1913 Webster]

WordNet (r) 3.0 (2006):

convenience
    n 1: the state of being suitable or opportune; "chairs arranged
         for his own convenience"
    2: the quality of being useful and convenient; "they offered the
       convenience of an installment plan" [ant: inconvenience]
    3: a toilet that is available to the public [syn: public
       toilet, comfort station, public convenience,
       convenience, public lavatory, restroom, toilet
       facility, wash room]
    4: a device or control that is very useful for a particular job
       [syn: appliance, contraption, contrivance,
       convenience, gadget, gizmo, gismo, widget]

Moby Thesaurus II by Grady Ward, 1.0:

145 Moby Thesaurus words for "convenience":
   WC, accommodate, accommodation, adaptability, advantage,
   advantageousness, advisability, amenity, appliance,
   appropriateness, appurtenance, auspiciousness, avail, backhouse,
   basement, bathroom, behalf, behoof, beneficialness, benefit, can,
   chargedness, closet, comfort, comfort station, commodiousness,
   coziness, crapper, criticality, crucialness, cushioniness, decency,
   desirability, drive, ductility, earth closet, ease, easiness,
   engine, enginery, expedience, expediency, facility, favor,
   favorableness, feasibility, felicitousness, fitness, fittingness,
   fixture, flexibility, free time, freedom, friendliness,
   fruitfulness, goof-off time, handiness, head, homelikeness,
   homeliness, homeyness, hospitality, idle hours, interest, john,
   johnny, johnny house, latrine, lavatory, leisure, loadedness, loo,
   luxuriousness, machine, machinery, malleability, manageability,
   manageableness, maneuverability, mechanical aid, mechanical device,
   mechanism, meetness, motive power, motor, necessary, odd moments,
   opportuneness, outhouse, peace, peacefulness, percentage,
   pliability, pliancy, point, politicness, powder room, power plant,
   power source, practicality, pregnancy, privy, profit,
   profitability, propitiousness, propriety, prudence, repose,
   reposefulness, rest, rest room, restfulness, retirement, rightness,
   ripeness, roominess, seasonableness, seemliness, semiretirement,
   service, snugness, softness, spare time, suitability, throne, time,
   time to kill, time to spare, timeliness, toilet, toilet room,
   untroublesomeness, urinal, use, usefulness, utility, value,
   warmness, washroom, water closet, wieldableness, wieldiness,
   wisdom, worth, worthwhileness