To be pedantic, the MW in OpenMW doesn’t stand for anything. It’s supposed to make players think of “Open MorroWind”, but only Bethesda has the right to use the name Morrowind.

Skywind: Playing the plot of Morrowind in the Skyrim engine. Game mechanics, spells, skills, models, textures, and bugs are all going to be similar to what you have in Skyrim, give or take some modding.

OpenMW: Playing Morrowind in an open source, crossplatform game engine. You are playing the exact same game as TES III (gameplay mechanics, spells, models, textures, etc are 1:1 identical in a vanilla install) but the code behind it is less buggy, more moddable, and it provides many modern quality-of-life improvements that the original engine couldn’t do.