The open-source, Unix-like operating system family built around a shared kernel and a POSIX-influenced toolset.
Linux is a kernel — a single program managing hardware, memory, and processes — that inherited its design philosophy almost entirely from Unix, the operating system built at Bell Labs in 1969. "Unix-like" means a system that behaves like Unix even if it isn't literally Unix's original source code: a hierarchical filesystem, everything-is-a-file where practical, multi-user permissions from day one, and small composable command-line tools.
"Linux/Unix Fundamentals" as a category covers the handful of ideas that stay constant no matter which distro, cloud provider, or decade you're in: processes have a parent, files have an owner and permission bits, the shell is just another program, and nearly everything is configured through plain text files you can read with cat.
The practical split that matters day to day is kernel vs. distribution. The Linux kernel itself only handles hardware abstraction, scheduling, and system calls — it has no package manager, no login prompt, no desktop. A "Linux distribution" (Ubuntu, RHEL, Debian, Alpine...) wraps that kernel with a init system, package manager, standard libraries, and defaults. Two distros can run the exact same kernel version and still feel completely different because of what's layered on top.
The other Unix-inherited idea worth internalizing early: almost every interaction with the system happens through the same small set of primitives — files, processes, and signals — which is why the same handful of tools (grep, ps, chmod) work identically whether you're on a Raspberry Pi or a thousand-node cluster.
fstab syntax, service names, and default shells vary by distro even though the kernel concepts don't.sed or date often differ.