The use of another’s work without citation or accreditation, with the intent of passing it off as one’s own.
Plagiarism is a type of copying, but is not necessarily copyright infringement. Therefore, it would be possible to have a situation in which use of someone else’s work was not a copyright infringement (the use was fair, the work was in the public domain, the user had permission) but was still plagiarism, because the user did not acknowledge the true author of the work in question. Although such a use would be legal, it would be unethical.
Copying and giving appropriate credit is not plagiarism, but could still be copyright infringement.