Plagiarism is the unauthorized use or close imitation of the language and thoughts of another author and the representation of them as one's own original work. (Wiki)
Now let's think of that definition in the context of authorship in science.
According to the study by Swazey, Anderson and Lewis the rates of plagiarism and inappropriate authorship were reported to be similar by both faculty and students. They found that inappropriate authorship was slightly more frequent than plagiarism with one the twist: While plagiarism was about three times more likely to be committed by students than by faculty, inappropriate authorship was about three times more likely to be committed by faculty than by students.
I think the National Academy of Sciences in the study of scientific misconduct should include the most frequent and important issue of possible misconduct relevant to the designation of authorship besides aspects like fabrication, falsification, and plagiarism.