I have seen many academicians use the terms Plagiarism and Similarity inaccurate. They simply use the Plagiarism for the Turnitin report.
Actually, the output of Turnitin is similarity index (similarity ratio). This comes from some sentences/words/names are analogical with other published materials (papers, books, internet, even submitted work). This doesn't mean there is a plagiarism. As the similarity ratio could be resulted from author names, affiliations, references, common expression,.....
On the other hand, the work (paper, book, ..) may contain plagiarism (larceny) even the turnitin report shows only