A periodical is a publication that comes out at regular intervals: daily, weekly, monthly, or quarterly. Newspapers are daily periodicals. National Geographic is a monthly periodical. Scientific journals that publish new research four times a year are quarterly periodicals.

What makes something a periodical is that it keeps coming, issue after issue, following a predictable schedule. Each issue contains new articles, stories, or information, unlike a book, which stays the same once it's printed.

Libraries often have entire sections devoted to periodicals. Some periodicals, like The Wall Street Journal or Scientific American, have been publishing for over a hundred years, creating an incredible record of how knowledge, events, and ideas have changed over time. Researchers studying history often turn to old periodicals to learn what people were thinking and talking about decades or centuries ago.

The word can also be an adjective meaning something that happens at regular intervals. A doctor might recommend periodical checkups every six months, or a building might need periodical inspections every year. In this sense, periodical emphasizes the recurring, scheduled nature of something rather than it being random or one-time.