Android App Bundle
The publishing format that replaced the APK on Google Play — the developer uploads everything, the store builds a tailored APK per device.
A traditional APK has to contain resources for every screen density, every language and every processor architecture, since you cannot know which device will download it. The AAB gathers it all into one package that Google Play splits and reassembles at download time. Downloads typically shrink by about a third.
The format has been mandatory for new apps on Play since August 2021. The contested consequence is that Google signs the final APK, and therefore holds the signing key — moving a degree of control from developer to platform. Outside Play, on F-Droid for instance, ordinary APKs are still used.