(
Super 
Audio 
CD)  A high-resolution audio format from Sony and Philips.  Designed as an optical disc format with higher-quality sound than a CD, SACD never caught on.  Neither did DVD-Audio, a competing next-generation disc format.  However, high-resolution music downloads are available in DVD-Audio resolutions as well as the DSD format used in SACD discs (see 
DVD-Audio, 
88.2/24, 
176.4/24 and 
high-resolution audio).
In 1999, Sony introduced the first SACD player.  The SCD-1's $5,000 price placed it squarely in the audiophile league, but subsequent players moved SACD well into the consumer realm, with combo CD/DVD/SACD/DVD-Audio units less than $200.
SACD Uses Direct Stream Digital (DSD)
Sony's SACD uses a 1-bit pulse density modulation (PDM) method that encodes 2,822,400 samples per second.  For more details, see 
DSD.
Multi-Channel and Hybrid Discs
Introduced in only stereo, 5.1 channel support was later added.  In 2003, hybrid SACD discs included a separate CD layer with the same titles for playback in regular CD players.
The DSD Approach
Sony's SACD audio format is a dramatic departure from PCM, the common way audio is sampled.  Direct Stream Digital (DSD) 1-bit samples are either 0 or 1 (up or down) depending on the previous sample point (see 
DSD).
This BDP-83 player from Oppo Digital not only played SACD and DVD-Audio discs, but regular CDs, DVDs and Blu-ray videos.  Oppo made a line of excellent players for more than a decade but streaming became too popular, and equipment manufacturers suffered as a result.  (Image courtesy of Oppo Digital, Inc.)
High-End Players Still Exist
There always seems to be a market for high-end audio equipment, because this PerfectWave SACD player from PS Audio was available in 2021 for $6499.  It uses the I2S interface if coupled with PS Audio's own DAC (see 
I2S and 
D/A converter).  
(Image courtesy of PS Audio Inc.)