Unix-like
A Unix-like operating system behaves similarly to Unix or follows its design principles without necessarily being certified as UNIX.
Unix: A family of operating systems originating at Bell Labs in the 1970s, known for portability and composable tools. Unix is the historical model whose interfaces and design principles Unix-like systems emulate.
Unix kernel: The privileged core of an operating system that manages hardware and provides services to programs. The kernel supplies the process, memory, and device services beneath a Unix-like system's user environment.
Bell Labs: The research and development organization where Unix was created in the late 1960s and early 1970s. Unix-like systems trace their design lineage to work carried out at Bell Labs.
UNIX certification: A formal conformance process that allows qualifying products to use the UNIX trademark. Unix-like describes resemblance or design lineage; certification is a separate, testable status.
FreeBSD: A free, open-source operating system descended from the Berkeley Software Distribution. FreeBSD is a prominent Unix-like system with a direct BSD lineage.
POSIX: A family of IEEE standards specifying operating-system interfaces, shells, and utilities for application portability. POSIX codifies interfaces commonly implemented by Unix-like systems, though not every such system conforms fully.
File descriptor: A small integer handle through which a process accesses an open file or another input-output resource. Standard input, output, and error use file descriptors, enabling Unix-style redirection and pipelines.
Ken Thompson: An American computer scientist who co-created Unix and designed the B programming language. Thompson helped create the original system that later operating systems emulated or extended.
Unix clone: An independently implemented operating system designed to reproduce Unix behavior or interfaces. A clone is one route to being Unix-like, unlike systems descended directly from Unix code.
OpenBSD: A free, open-source BSD operating system known for security-focused engineering. OpenBSD applies Unix-like interfaces and tools with a strong emphasis on code auditing and secure defaults.