A periodical is a publication that comes out periodically. A synonym is "serial." Magazines, newspapers, and journals are all types of periodicals. They may come out daily, weekly, monthly, quarterly or annually, but new issues are released during a specific period. Comic books issues also count as a periodical.
Periodical issues are indexed by volume and year (these numbers factor into the citation of an article as well) within a database. Databases do not contain every periodical title, but rather a specific set usually, depending on their arrangements with the periodical publisher(s). Sometimes, one database will only offer limited coverage (dates or timeframe) of a title, but not every single issue ever released. Because of embargoes, you might not see the most recent articles of an issue available in the databases, because publishers want to drive direct subscriptions to their content. You can see which database holds a periodical title and its coverage availability here.
