Is Cinnamon a desktop environment or a window manager? (Or both?)

Cinnamon is a desktop environment, as its wikipedia page and archwiki page both state.

Cinnamon uses its own window manager called muffin, installing the cinnamon package also installs the muffin WM package on Debian.

Cinnamon also has "a bundle of programs running on top of a computer operating system, which share a common graphical user interface (GUI)" as Wikipedia's Desktop environment says as a basic definition. Its programs are X-Apps, but like all programs they're generally voluntary if you want to remove & use others instead.

Additionally, the archwiki page also states:

Cinnamon does not support using a different window manager.