I think it's basically a stateless PaaS/microservice with a set of constrains/APIs on top. The BaaS/Serveless architectures also seem to have their own sub-categories based on the constrains/target. Some are fat(i.e. a kind of full blown PaaS with a common/advanced API gateway on top like parse.com) and others are lightweight(i.e. amazon lambda) meant to run as a function(e.g. update a record on event changes). In the end they are various abstractions that hopefully will improve the security and help you to manage your infrastructure easier.