I'm a native US speaker. The following is my opinion. Alteration is a change in pitch from the base pitch. Base pitch is C, alteration is sharp, actual pitch is C#.
Accidental is a change in pitch from the standard scale pitch. As mentioned by Peter, C# in a D major scale is not an accidental, although it is an alteration. I would totally support cleaning up this in NR 1.1.1 Accidentals (Note -- we don't have 1.1.1.4 in the NR; the lowest level of headings is not numbered). Carl https://codereview.appspot.com/579280043/
