Intentional plagiarism occurs when a student uses someone else’s work and claims it as his or her own work even though he or she knows that this is plagiarism. It is not accidental, it is purposeful.
In the case of intentional plagiarism, there is no simple mistake on the part of the student. The student knows they are copying someone else’s work and saying that it is their own. They are aware that this constitutes plagiarism. Even so, they use the work, presumably because they do not want to do the work themselves and they do not believe that they will get caught.