aufs (short for advanced multi-layered unification filesystem) implements a union mount for Linux file systems. The name is an acronym which originally stood for AnotherUnionFS, but since version 2 it stands for "advanced multi-layered unification filesystem".
Developed by Junjiro Okajima in 2006, aufs is a complete rewrite of the earlier UnionFS. It aimed to improve reliability and performance, but also introduced some new concepts, like writable branch balancing, and other improvements – some of which are now implemented in the UnionFS 2.x branch.
aufs was rejected for merging into mainline Linux. Its code was criticized for being "dense, unreadable, and uncommented". Instead, OverlayFS was merged in the Linux kernel. After several attempts, the author has given it up.