A mirror tournament (or mirror) is a tournament where the host obtains the question set from a different tournament ("mirroring") rather than writing the questions itself. Such a tournament can obtain packets from a housewrite or a vendor like NAQT.
A mirror is contrasted with the main site, which is the tournament run by the school which wrote the set. However, the meaning of the term has largely expanded to mean "any site where a question set is used," including the main site.
For example: the 2013 Yale BHSAT tournament had a main site at Yale, which could or not be called a mirror site. Other tournaments outside Yale which agreed to use the set, such as ATROPHY at Northern Illinois University, are "mirrors" of BHSAT because they are "mirroring" the same set of questions.
In the strictest definition of a mirror, the host team does no editing or writing (and, if it's a college event, participating teams do not submit packets).
In the current era, the majority of tournaments are mirrors because most sets are either housewrites or provided by a company like NAQT or ACF. Most sets will attempt to have at least one school in each major region of the country run a tournament using their set.
This was not always the case. For much of the history of quiz bowl, packet-sub was the dominant means of producing tournaments. As a result, most sets were tied to a specific tournament, as the teams that were attending that specific site had contributed to producing the set. Mirrors of these events employed a looser definition of the term, as they often required teams to submit packets.
When American sets are mirrored internationally in Canada and the UK (or vice-versa), there may be country-specific modifications to the set. This is especially true of whichever set is chosen for the year's BSQC.
The nationwide regional tournaments ACF Fall, ACF Regionals, and NAQT Sectionals (and formerly TRASH Regionals) are not traditionally considered mirror tournaments, since the editing comes from a single centralized source not directly affiliated with any particular regional tournament. However, schools that host these tournaments are often known as mirror sites.
For tournament mirrors, the question writers usually charge $10-$20 for each team participating in a high school set mirror, and $40-$50 for each team participating in a collegiate set mirror. About 25% of a mirror's revenue is paid back to the original producers of the set.