"The practice of taking someone else's work or ideas and passing them off as one's own" - Oxford Dictionaries online
It is possible to plagiarize your own work--this is known as self-plagiarism. Self-plagiarism happens when you copy or use your own ideas from prior work or assignments.
Colleges and universities include plagiarism in their student codes of conduct, and there are consequences that range depending on severity.
"They Say / I Say" (Graff & Birkenstein, 2014) is an approach to academic writing that helps you be clear about which ideas are your own (I say), and which ideas are the ideas of others (They say).
This template from Graff & Birkenstein's book demonstrates some of these strategies (2014).
The general argument made by author X in [her/his/their] work, _______________, is that _______________ [(Use in text citations as needed)]. More specifically, X argues that _______________. [She/he/they] writes, “ _______________” [(Quotes always require in text citations)]. In this passage, X is suggesting that _______________. In conclusion, X’s belief is that _______________.
In my view, X is wrong/right, because _______________. More specifically, I believe that _______________. For example, ___________. Although X might object that __________, I maintain that _______________. Therefore, I conclude that _______________.