WINE is a project to allow a computer running a Unix-like operating system to run programs designed for the Microsoft Windows API. The name was derived from the recursive acronym "Wine Is Not an Emulator." While this is technically true, to many of its users Wine behaves much like an emulator, leading to the alternative expansion "WINdows Emulator." Wine is free software. It was originally under the same MIT license as X11, but, owing to concern about proprietary versions of Wine not contributing changes back to the core project, work as of March 2002 is licensed under the LGPL.
The Wine project began in 1993 and is still in development. It originally targeted Windows 3.x (16-bit) applications. The present focus is primarily on the dominant Win32 (32-bit) applications. The project probably originated in discussions on Usenet in comp.os.linux, and the first working code was created by Eric Youngdale and Bob Amstadt. Alexandre Julliard has been the project leader since 1994.
Instead of acting as an emulator, Wine implements a compatibility layer, which provides an alternative to the Windows-specific DLLs that Windows programs call. Alternately, those wishing to port a Windows application to a Unix-like system can compile it against the Wine libraries.
The project has been time-consuming and difficult for developers, at least partially because of incomplete documentation of the Win32 API. While most Win32 functions are documented, there are areas such as file formats and protocols where an official Microsoft specification does not exist, as well as undocumented low level functions and obscure bugs that must be duplicated precisely for some applications to work properly. Consequently, the Wine team have had to reverse engineer many function calls and file formats, in such areas as thunking.
As of early 2004, Wine runs many well-known programs, such as Lotus Notes and some versions of Microsoft Office, with varying levels of accuracy and stability. If the user includes native Microsoft Windows DLLs from their previous Windows installation, the number of applications which can be run successfully greatly increases.
The involvement of Corel for a time helped the project, chiefly by employing Julliard and others to work on it. This help was motivated by Corel's porting of its own suite of office programs to Linux. However, the effort has now stopped due to Corel's financial difficulties.
Other organizations have made efforts to commercially support Wine. CodeWeavers markets a proprietary version called CrossOver Office specifically for running Microsoft Office and other major Windows applications. TransGaming Technologies produces the proprietary WineX, tailored towards running Windows games and available on a subscription basis. The core Wine development is more directed towards a correct implementation of the Windows API as a whole and lags somewhat behind in these areas.