Machine, mechanism, machinery, apparatus, engine, motor are comparable especially when they denote a device or system by which energy can be converted into useful work.
Machine is at once the most fundamental of these terms and the most varied in its applications. Basically it denotes an assemblage of parts that transmit forces, motion, and energy from one to another in a predetermined manner and to a desired end (as sewing a seam, hoisting a load, printing a book, or maintaining an electric current).
But it also may apply specifically to any of the six simple machines (the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the screw) that together contain the elements of which all other machines are composed. Again, it may apply to a machine in the basic sense together with its power-generating unit and sometimes with supplementary equipment (as for moving the whole complex) or it may apply specifically to a conveyance and especially an automobile and machine is the only term of this group that is freely used collectively of machines as a class or abstractly of the technology and technological society associated with their use.
Mechanism may come close to the basic sense of machine when it denotes an assemblage of working parts functioning together to produce an effect, but more often than not it applies to relatively simple straightforward mechanical linkages such as make up a complex machine; thus, a sewing machine is made up of several mechanisms (as one to advance the thread, another to convey the cloth, another to determine the length of the stitch, and still another to wind the bobbins).
Sometimes, however, mechanism suggests not merely the physical parts but the various steps that lead to the final result of the process.
Machinery (see also EQUIPMENT ) may apply to machines collectively, but it may also replace mechanism to denote an assemblage of working parts performing a function.
Apparatus (see also EQUIPMENT ) basically denotes an assemblage of parts for attaining some end or doing some thing, but in itself it implies nothing about the complexity or simplicity, the efficiency or inefficiency, or the precision or crudity of the assemblage; thus, chimpanzees have been reported to put sticks together into a crude apparatus for reaching fruit that is beyond the reach of their hand.
Engine and motor in their basic relevant sense both denote a machine for converting energy (as heat, chemical, or nuclear energy) into mechanical force or motion, but in many situations they are not at all interchangeable, choice between them being firmly fixed by idiom.
Engine is the more general term in this relation and is applicable to such machines whether large or small, simple or complex or it may apply both to a power-generating unit and a working unit that depends upon this and sometimes specifically designates certain automotive units (as a locomotive or fire engine). From an earlier general use engine is still specifically applied to a few kinds of machines and it is the idiomatically appropriate term to designate the power plant of an aircraft.
Motor is applicable to a small or light engine or to a gasoline or other internal-combustion engine; thus, one may speak of the motor or engine of an automobile; diesel engines or motors power many modern locomotives.
Motor is the specific term for a rotating machine that transforms electrical energy into mechanical energy. Like machine, motor also applies specifically to an automotive vehicle (as an automobile or truck).