Definition - What does Cygwin mean?
Cygwin is a command line interface environment similar to Unix, but designed for Microsoft-based platforms. Cygwin allows the development and testing of Windows-based applications on a Unix-like platform. Thus, Unix applications can be developed in a Cygwin environment to be executed easily on the Windows platform with little original source code alteration. In addition, Windows applications could be launched from within Cygwin as well as using Cygwin tools and applications in the context of Windows systems.
Cygwin uses Microsoft’s GNU library, which provides the Portable Operating System Interface (POSIX) for Unix system calls and procedures.
Techopedia explains Cygwin
Cygwin is a development environment for the Microsoft Windows family of operating systems that aids in the development of applications in an environment similar to Linux/Unix through library sets and tools. Many Linux, Unix, GNU and BSD programs and packages are included in Cygwin.
Cygwin uses a dynamic-link library file that provides Linux application programming interface (API) functionalities, such as POSIX system calls. POSIX is a set of APIs, shell, and other utilities that support Unix application compatibility. Although Cygwin mainly relies on POSIX APIs to function, it comes with a number of tools and utilities that provide the look and feel of Linux/Unix.