Here's why:
* Periodicals are publications that are issued at regular intervals, like weekly, monthly, or quarterly. Examples include magazines, newspapers, and journals.
* Dictionaries are reference books that provide definitions of words and other information, such as pronunciations, etymologies, and usage examples. They are typically published once and updated periodically with new editions.
While a dictionary might be updated with new editions, it's not published at regular intervals like a periodical. Therefore, it's not considered a periodical.