Audio Units (AU) are a system-level plug-in architecture provided by Core Audio in Mac OS X developed by Apple Computer. Audio Units are a set of application programming interface 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 the other popular plug-in format, Steinberg's 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).
Mac OS X comes with Audio Units allowing one to timestretch an audio file, convert its sample rate and stream audio over a Local Area Network. It also comes with a collection 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, Final Cut Pro, MainStage and most 3rd party audio software developed for Mac OS X such as Ardour, Ableton Live, REAPER and Digital Performer.
Competing technologies 
- Digidesign's Real Time AudioSuite
- LADSPA, DSSI and LV2 for Linux
- Microsoft's DirectX
- Steinberg's Virtual Studio Technology
- Apple's Audio Unit Programming Guide
- Apple's Core Audio Overview
- Apple's Audio Units trademark and license agreements
- Tips on how to use Audio Units and which hosts that are available etc.
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