Academic plagiarism has a formal definition, but to state it informally simply it is using someone else’s work as ones own.
It may be unintentional, merely a footnote left out, or missing quotation marks or forgetting these indicators may be a clever way to fool the reader into thinking the material is original to the author.
It is illegal, and unethical, but there seems to be an upward trend in this activity. According to recent studies here is what was determined about the rise in plagiarism