ADX is an audio codec created by CRI Middleware Co. Ltd., and was originally released in 1996.
ADX is a multi-streaming system, providing audio compression features while minimising processor load. It was used in video games to deliver relatively high quality audio without the need to stream directly from a disc. It is a 4-bit ADPCM format and can store mono or stereo audio, and is capable of looping audio (or playing from any arbitrary point in the audio's data) without noticable stutters (as would be the case, for example, with Redbook audio streamed in real time from a disc).
Sega's close relationship with CRI meant that ADX became a standard within the company for many years, debuting in around 1998 in Sega Saturn games and the vast majority of Sega Dreamcast ones. ADX has also been used by Sega since becoming a third-party developer. CRI also combined the technology with MPEG streams to create the Sofdec movie codec.
Today the technology is obsolete, with CRI launching a successor in ADX 2.