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Aperiodical is any item that gets published routinely by a set period of time. It is published "period"-ically. Get it??
Works that got published ONCE A DAY were newspapers.
Works that got published ONCE A WEEK or ONCE A MONTH were magazines.
Works that got published ONCE PER QUARTER (four times a year--once in the winter, once in the spring, once in the summer, once in the fall) were scholarly journals.
Those cycles have changed a bit now that we live in the Internet Age and information can be published as soon as someone inputs it onto a web page. So newspaper items may be published via online websites (such as the Los Angeles Timesor the New York Times) at any time throughout the day; same with items published on magazine web sites (such as Newsweek.comor Time.com).
Even so, we continue to consider all the above-mentioned items--newspapers, magazines, and journals--to qualify under a large conceptual umbrella as periodicals.