Wine is a free and open-source compatibility layer that aims to allow application software and computer games developed for Microsoft Windows to run on Unix-like operating systems. Wine also provides a software library, named Winelib, against which developers can compile Windows applications to help port them to Unix-like systems.

Wine provides its compatibility layer for Windows runtime system (also called runtime environment) which translates Windows API calls into POSIX API calls, recreating the directory structure of Windows, and providing alternative implementations of Windows system libraries, system services through wineserver and various other components (such as Internet Explorer, the Windows Registry Editor, and msiexec). Wine is predominantly written using black-box testing reverse-engineering, to avoid copyright issues.

The selection of "Wine is Not an Emulator" as the name of the Wine Project was the result of a naming discussion in August 1993 and credited to David Niemi. There is some confusion caused by an early FAQ using Windows Emulator and other invalid sources that appear after the Wine Project name being set. No code emulation or virtualization occurs when running a Windows application under Wine. "Emulation" usually would refer to execution of compiled code intended for one processor (such as x86) by interpreting/recompiling software running on a different processor (such as PowerPC). While the name sometimes appears in the forms WINE and wine, the project developers have agreed to standardize on the form Wine.

Wine is primarily developed for Linux and macOS, and there are, as of July 2020, well-maintained packages available for both platforms.

In a 2007 survey by desktoplinux.com of 38,500 Linux desktop users, 31.5% of respondents reported using Wine to run Windows applications. This plurality was larger than all x86 virtualization programs combined, as well as larger than the 27.9% who reported not running Windows applications.