The Office of Research Integrity, Public Health Service, U.S. Department of Health and Human Services "considers plagiarism to include both the theft or misappropriation of intellectual property and the substantial unattributed textual copying of another's work. It [plagiarism] does not include authorship or credit disputes."
See http://ori.dhhs.gov/policies/plagiarism.shtml

While most in academia are well aware that using direct quotes without attribution is plagiarism, it is important to recognize that not all students are aware of this point, particularly when it comes to information downloaded from the Internet (as opposed to that obtained from traditional print forms). Special attention should be paid to students whose primary language is not English. Because of the difficulty some have with English as a second language, they may be tempted to borrow phrases or whole sentences. Furthermore, even if the temptation is not there, they may not understand what constitutes a reasonable paraphrase.

Authorship or credit disputes may arise among collaborators, particularly after the collaboration has ended. During a collaboration, all parties expect to have access to the end products of the collaboration, and hence it is not reasonable to view the misuse of such a product as theft by one collaborator from another. It is inappropriate, however, to use the product of joint work without proper attribution to all collaborators.