Audio Units (AU) are a system-level plug-in architecture provided by Core Audio in Apple's macOS and iOS operating systems. Audio Units are a set of application programming interface (API) services provided by the operating system to generate, process, receive, or otherwise manipulate streams of audio in near-real-time with minimal latency. It may be thought of as Apple's architectural equivalent to another popular plug-in format, Steinberg's Virtual Studio Technology (VST). Because of the many similarities between Audio Units and VST, several commercial and free wrapping technologies are available (e.g. Symbiosis and FXpansion VST-AU Adapter).
Audio Units allows sound file audio time-scale/pitch modification (e.g., timestretch), sample rate conversion, and streaming over a Local Area Network. It also comes with a set of AU plug-ins such as EQ filters, dynamic processors, delay, reverb, and a Soundbank Synthesizer Instrument.
AU are used by Apple applications such as GarageBand, Soundtrack Pro, Logic Express, Logic Pro, Logic Pro X, Final Cut Pro, MainStage and most 3rd party audio software developed for macOS such as Ardour, Ableton Live, Amadeus Pro, DaVinci Resolve, REAPER and Digital Performer.