D is for… Demoscene
The demoscene kicked off around the mid-‘80s on 8-bit machines, most notably the C64. It was very much tied to the cracker scene of the time; cracked versions of games (ie, with copy protection removed, and sometimes with cheats added, etc) often featured an 'intro' bigging up the cracker group responsible, often accompanied by some pretty graphics and a nifty bit of chip music.
Eventually, the 'intro' concept spawned full-length, standalone pieces called demos. Even as the demoscene moved onto more powerful computers like the Commodore Amiga and later the PC, chiptunes were still used as they could be stuffed into very small amounts of computer memory.
D is also for... Digitised sounds
Back in the day, sampled sounds were often described as "digitised", and some chip music uses digitised sounds, often for the drums.