A person is having a readathon when they are reading a book series from start to finish without reading other books in between or taking long breaks. In addition to book series readathons, one can also have author and genre readathons.
Boy: I finished Harry Potter and the Deathly Hallows last night, and with that my Harry Potter readathon!

Girl: You're such a geek. I still have two books to go.
by Tinface August 4, 2009
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Time spent reading two or more books in one sitting with minimal to no breaks in between. Usually during a set period of time.
I heard you were joining the twenty-four hour Readathon planned for next week, what books do you plan to read?
by Harleyl November 14, 2017
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