Raspbian [1] is the name of the original community project to port Debian to armhf (armv6 hard float) which ran on the Raspberry Pi 1.
Raspberry Pi put our their own images based on this, and called it Raspbian until about 2020 [2] when they started calling it "Raspberry Pi OS" after they started producing aarch64 images.
Raspberry Pi put our their own images based on this, and called it Raspbian until about 2020 [2] when they started calling it "Raspberry Pi OS" after they started producing aarch64 images.
[1] https://www.raspbian.org/
[2] https://www.raspberrypi.com/news/8gb-raspberry-pi-4-on-sale-...
Further reading:
[4] https://www.tomshardware.com/news/raspberry-pi-os-no-longer-...
[5] https://unix.stackexchange.com/a/602658/253655