The term 'arrangement' has become a casually used metaphor for the rendering of one version of a piece of music into another. I use this term, admittedly erroneously, when giving credit in my publications to the person who has 'dished up' (to borrow Percy Grainger's coinage) a tune for brass.
This isn't really fair to arrangers. Arranging is a very creative endeavor, just short of the pinnacle of setting notes to paper - composition. Nelson Riddle, Henry Mancini, John Williams all got their start in the 'biz' first as session piano players, then as arrangers and orchestrators, and then the fabulous composers we know them to be.
So, to be clear, most of what comprises our catalog is orchestration - the rendering of a keyboard work for other instruments. A few charts in the catalog are transcriptions - the rendering of a work originally written for orchestra (or band, or other such combination of instruments) for a different combination of instruments. Orchestration is creative in its own way, but rarely entails the manufacture of original material.
I prefer orchestration to transcription. Even when making a transcription (which I am generally reluctant to do), I will seek out the composer's piano version of his own work if available to avoid trying to make a trumpet sound like an oboe or other such nonsense - it doesn't work that way and it just pisses off the trumpet player (and the oboist).
To take this thought a step further, there are some works that, in my opinion, should not be touched - the Stravinsky ballets for example are so perfectly conceived as orchestral works that to try to make them 'work for brass' is the height of hubris.