"Alien space bats" originally was used as a sarcastic attack on poorly written alternative histories due to lack of plausibility. These attacks are usually phrased as the need for "alien space bats" or by saying the alternative history has gone into "ASB territory". The term eventually evolved into a reference to deus ex machina to create an impossible point of divergences. Examples include changes to the physical laws of nature, introducing magic into the world, time travel, and advanced aliens interfering in human affairs. An example of the latter is Harry Turtledove's Worldwar series.
The term "alien space bats" was first coined, then popularized in the Usenet group soc.history.what-if. Alison Brooks (1959-2002) is credited as the creator of the term, using it to debunk the possibility of a successful Operation Sea Lion by saying the only way it could be successful was if alien space bats helped the Nazis. Brooks regretted the use of the ASBs as a supernatural agency, preferring to restrict them to rhetoric.
S. M. Stirling credited Brooks with creating the term in the acknowledgments section of Dies the Fire, in which he changed the laws of physics, and also used the plot device to send Nantucket back in time in Island in the Sea of Time. One character throughout Dies the Fire and its sequels believes the change to the laws of nature was done by an advanced alien race because the changes were finely tailored, and refers to this race as alien space bats. In a review of Dies the Fire, Dale Cozort addressed the perceived implausibility of the novel by saying "Just say to yourself, 'The elder gods or alien space bats took our toys away and that's all there is to it.'"Paul Di Filippo uses the term often when reviewing the series. The term also appeared in John Birmingham's 2008 novel Without Warning.