PSID
The file format that packs a C64 tune as tiny machine code and lets an emulator play it back on any computer.
A .sid file in PSID format holds no recorded audio but the player routine itself: the 6510 machine code and data the original tune was made of, plus a short header with title, author and load address. A SID player runs that code on an emulated 6510 and mimics the SID chip, so the music is recreated note by note rather than streamed.
The format grew out of the SIDPlay community in the 1990s and became the standard for archiving C64 music; the HVSC collects tens of thousands of tunes as PSID files. The RSID variant is stricter and demands a more complete C64 environment — for tunes that rely on hardware timing the simpler PSID makes no promises about.