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

Wine is a compatibility layer. It duplicates functions of Windows by providing alternative implementations of the DLLs that Windows programs call, and a process to substitute for the Windows NT kernel. This method of duplication differs from other methods that might also be considered emulation, where Windows programs run in a virtual machine. Wine is predominantly written using black-box testing reverse-engineering, to avoid copyright issues.

The name Wine initially was an acronym for WINdows Emulator. Its meaning later shifted to the recursive backronym, Wine Is Not an Emulator in order to differentiate the software from other emulators. While the name sometimes appears in the forms WINE and wine, the project developers have agreed to standardize on the form Wine.

The phrase "Wine Is Not an Emulator" is a reference to the fact that no processor code execution emulation occurs when running a Windows app under Wine. "Emulation" usually refers to the execution of compiled code intended for one processor (say, x86) by interpreting/recompiling software running on a different processor (say, PowerPC). Such emulation is almost always much slower than execution of the same code by the processor for which the code was compiled. In Wine, the Windows app's compiled x86 code runs at full native speed on the computer's x86 processor, just as it does when running under Windows. And Windows API calls and services also are not emulated, but rather substituted with Linux equivalents that are compiled for x86 and run at full, native speed.

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.