Linux® is an open source operating system (OS). An operating system is the software that directly manages a system's hardware and resources, like CPU, memory, and storage.
The OS sits between applications and hardware and makes the connections between all of your software and the physical resources that do the work.
Hmmm, actually Linux
is just a free/libre/open source operating system kernel.
are complete, open-source operating systems that combine the Linux kernel with many other pieces of free-software - e.g. GNU userland tools, GNU C runtime, a compositing/windowing system (X11, and or Wayland), login manager, desktop environment/window manager, plus a selection of other useful software (browsers, editors, media players etc).
Depending on each Linux distributions policies and ethics, some also include non-free software - e.g. software that is free of financial cost, but is NOT open source and therefore not truly free software (Not free as in freedom!).
Others completely exclude non-free software and do not give their users easy access to non-free software.
Just thought I'd clear that up!
Linux = OS kernel
Linux distribution = complete OS featuring Linux kernel + lots of other software.