A periodical is a magazine or journal that is published periodically throughout the year (i.e., weekly, monthly, six times a year, etc.). 

The Abridged Readers' Guide to Periodical Literature  is an alphabetized author/subject index to periodicals commonly used for reference. If there is an article about your topic listed in this guide you will be provided with the following information: the title of the article, the name of the periodical, the publication date, and the page number on which the article can be found.
 

World Almanac Indexes The Abridged Readers' Guide to Periodical Literature
Periodicals on hand at RMS Available National Geographic Magazine RMS Media Center
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