According to USC's current Student Handbook [p. 9] plagiarism is:
An allegation of plagiarism is intent-neutral. In other words, the reader cannot discern whether the absence of a citation was done deliberately or you simply forgot to add a citation. Therefore, it is important to proofread your paper before you submit it to ensure that all sources used during your research are cited and that every in-text citation is included and refers to the correct, fully cited source in your list of references. This is why it is important to keep track of everything you have used while researching and writing your paper, so you can easily correct any omissions.
If you have any doubts about whether to cite a particular argument or statement, protect yourself from an allegation of plagiarism by citing the source or sources that support your statements. This will help the reader determine the validity of your work. Note that, most importantly, a citation error not only can raise concerns about the academic integrity of your paper, but it tells the reader that you did not conduct an effective or thorough review of the literature. A missing citation also inhibits the reader's ability to review the source in order to obtain further information about what is being discussed in your paper and the arguments that you have presented.
Academic Integrity. The Writing Center. University of Kansas; Avoiding Plagiarism. Academic Skills Program, University of Canberra; How and When to Cite Other People's Work. Psychology Writing Center, University of Washington; Proctor, Margaret. "How Not to Plagiarize." University College Writing Centre. University of Toronto; Plagiarism. The Writing Center. University of North Carolina; Plagiarism. The Writing Center. Department of English, George Mason University. Avoiding Plagiarism. The Writing Lab and The OWL. Purdue University; Understanding and Avoiding Plagiarism. Writing@CSU. Colorado State University.