Wine for Mac (originally an acronym for 'Wine Is Not an Emulator') is a compatibility layer capable of running Windows applications on several POSIX-compliant operating systems, such as Linux, macOS, & BSD. Instead of simulating internal Windows logic like a virtual machine or emulator, Wine for macOS translates Windows API calls into POSIX calls on-the-fly, eliminating the performance and memory penalties of other methods and allowing you to cleanly integrate Windows applications into your desktop.
Note: this listing is for the official release of Wine, which only provides source code. If you want a version of Wine that is packaged specifically for macOS, then use WineBottler!