The Collaborative International Dictionary of English v.0.48:

Convertible \Con*vert"i*ble\, a. [L. convertibilis: cf. F.
   convertible.]
   1. Capable of being converted; susceptible of change;
      transmutable; transformable.
      [1913 Webster]

            Minerals are not convertible into another species,
            though of the same genus.             --Harvey.
      [1913 Webster]

   2. Capable of being exchanged or interchanged; reciprocal;
      interchangeable.
      [1913 Webster]

            So long as we are in the regions of nature,
            miraculous and improbable, miraculous and
            incredible, may be allowed to remain convertible
            terms.                                --Trench.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Automobile \Au"to*mo*bile`\, n. [F.]
   a self-propelled vehicle used for transporting passengers,
   suitable for use on a street or roadway. Many diferent models
   of automobiles have beenbuilt and sold commercially,
   possessing varied features such as a retractable roof (in a
   convertible), different braking systems, different
   propulsion systems, and varied styling. Most models have four
   wheels but some have been built with three wheels.
   Automobiles are usually propelled by internal combustion
   engines (using volatile inflammable liquids, as gasoline or
   petrol, alcohol, naphtha, etc.), and sometimes by steam
   engines, or electric motors. The power of the driving motor
   varies from under 50 H. P. for earlier models to over 200 H.
   P. larger models or high-performance sports or racing cars.
   An automobile is commonly called a car or an auto, and
   generally in British usage, motor cars.

   Syn: car, auto, machine, motorcar.
        [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

convertible
    adj 1: capable of being exchanged for or replaced by something
           of equal value; "convertible securities" [syn:
           convertible, exchangeable] [ant: inconvertible,
           unconvertible, unexchangeable]
    2: designed to be changed from one use or form to another; "a
       convertible sofa"; "a convertible coupe"
    3: capable of being changed in substance as if by alchemy; "is
       lead really transmutable into gold?"; "ideas translatable
       into reality" [syn: convertible, transformable,
       translatable, transmutable]
    n 1: a car that has top that can be folded or removed
    2: a corporate security (usually bonds or preferred stock) that
       can be exchanged for another form of security (usually common
       stock) [syn: convertible, convertible security]
    3: a sofa that can be converted into a bed [syn: convertible,
       sofa bed]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "convertible":
   all one, all the same, changeable, coequal, commutable,
   commutative, coordinate, correspondent, corresponding, equal,
   equalizing, equiparant, equipollent, equivalent, even, exchanged,
   give-and-take, identical, interchangeable, interchanged, liquid,
   modifiable, much the same, mutual, negotiable, permutable,
   reciprocal, reciprocating, reciprocative, resolvable, retaliatory,
   returnable, standard, swapped, switched, tantamount, traded,
   transformable, transitional, transmutable, transposed