In a debate, a bookstop is a requirement issued by one debater to another that they must read some external matter before being able to understand an argument. The term is also used with regard to other media such as videos.
When formally debating on Issuepedia, bookstops are forbidden by the clarity rule: it is the speaker's responsibility to clarify their argument, not the responder's responsibility to understand it.
To the best of my recollection, the term originated with the LessWrong community, but I cannot find any references to it on that site. Possibly I came across it in a dialogue in the #LessWrong channel on IRC, so it may have originated somewhere else entirely. --Woozle (talk) 12:07, 11 March 2013 (EDT)