These words all mean to get rid of something that is no longer needed or wanted. Discard , when applied to physical objects, emphasizes the action of casting aside or throwing out: to discard an old pair of shoes. It stresses the worthlessness of the object so disposed of, and thus, when applied by extension to a person, conveys a large measure of contempt. When applied to ideas, discard suggests a strong and usually permanent rejection: The recommendation that nuclear weapons be used was discarded as reckless and uncalled-for.

Junk , an informal term, means to discard as rubbish: to junk an old car. It implies that the thing discarded can no longer be used for its original purpose but that its materials may still be used. The metal and some parts of a junked car, for instance, can be re-used. Scrap is similar to and in more common use than junk in suggesting that the materials of something no longer serviceable can be re-used, but scrap is used almost exclusively of metal things and usually of large structures: to scrap an old World War II battleship. Scrap does not necessarily imply that the object discarded is worn out; it may simply be obsolete: the supersonic VT-705 jet fighter plane was scrapped in favour of a still faster and more manoeuvrable model. Unlike junk scrap often suggests that the thing discarded will or might be replaced by something else, or that the putting aside is a temporary expedient: the plan for slum clearance had to be scrapped for lack of funds but it is hoped the new government will restore the necessary funds.

Shed means to cast off by a natural process, as hair, skin, etc.: snakes shed their skins periodically; trees shedding their leaves in autumn. The idea of a protective covering is retained when the word is applied figuratively to human beings: to shed warm clothing when summer arrives; to shed one’s inhibitions after a few drinks. It here suggests getting rid of something unwanted but the thing rejected is often of such a nature that it cannot be discarded entirely or permanently. Jettison is also a discarding or throwing-away process, but almost always as a matter of necessity, as when fuel or cargo is jettisoned from a plane or other vessel to lighten the load.