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Peer-reviewed (PR) articles are also sometimes called scholarly articles, refereed articles, or research articles. PR articles are original-research articles published in PR journals, also sometimes called scholarly journals.
One complication is the distinction between the journal itself being peer-reviewed vs individual articles within it (eg. editorials, letters to the editor). If the article is a research article, describing an original study of some kind, and it's in a peer-reviewed journal, then it's safe to say the article itself is peer-reviewed.
To find peer-reviewed journals: search OneSearch and use the "Peer-reviewed" limit checkbox on the far left.
Other subject-specific databases will usually also have a peer-reviewed limiter of some kind. Medline is almost all peer-reviewed articles, so there is no limiter needed.
Peer-reviewed journals are distinguished from other kinds of periodicals by the content, authorship, and editorial process they go through to get published. Popular magazines and trade journals have articles that are typically written by professional writers who may or may not be experts in the topic they write about. Editors typically decide the topics and assign them to writers, or solicit articles from outside experts. The key is that the articles do NOT undergo review by comparable experts prior to being published. Peer-reviewed articles are written usually by university professors describing original research they have done in their own field of expertise. The editors of the journal seek other experts in the same field to read the manuscript and advise the editors as to whether the quality of the article meets that field's standards, before it gets published, hence the term "peer review". Peer-reviewed articles almost always have an abstract summarizing the article and an extensive list of references at the end documenting all of the prior research used and cited in the article, which provides a kind of history of the development of ideas and findings on the topic. The author's institutional affiliation (what university they work for, and usually what specific department) is almost always provided as well. Many peer-reviewed articles have more than one author; some in the physical sciences can have dozens of co-authors.