Alien Space Bats is a tongue-in-cheek term coined by Alison Brooks, who once said that the only way the Germans could win Operation Sealion was if they had the assistance of "alien bats from outer space."

ASBs have gone through some semantic drift since then, from meaning "wildly implausible" to "explicitly impossible." If your PoD involves the introduction (or reintroduction) of magic, demonic crosstime invasions, the first outbreak of the zombie virus? You're dealing with ASBs.

Just because ASBs are impossible doesn't mean that they're implausible. Once they're introduced, plausibility is a matter of writing: how people respond to them, how they think the ASBs work, how the ASBs do work, how well (and how poorly) they're implemented, and so forth.