A plug-in standard included by Apple in its OSX operating system. Audio Units (AUs) is intended to replace manufacturer-specific plug-in standards such as VST or MAS on Apple systems. In theory, all sequencer software and plug-ins written for OSX will support this standard, and plug-in authors will no longer have to write multiple versions for different standards. In current practice, most Macintosh plug-ins currently are offered in both AU and at least one other format, usually VST or RTAS.
In October 2012, Apple announced that the next version of GarageBand, its low-end DAW software, will required that AU plug-ins be able to run "sandboxed", which means that their access to system resources will be restricted. This created considerable consternation among plug-in developers, who feared that Logic, Apple's professional DAW package, would go the same route, and there were rumors that this was a prelude to requiring all AU plug-ins to be sold through Apple's App Store, which would give Apple a significant cut of the developer's sales revenue. Apparently at least one major plug-in software house threatened to stop supporting the AU standard if Apple went this route, and Apple backed off. As of 2017, Logic does not require plug-ins to be sandboxed, although Garageband does, and neither requires plug-ins to go through the App Store.