NeXTSTEP or NeXTSTEP/Sun depending on who you are asking.
Sun had a fairly large investment in OpenStep for a while. Before Sun entered the picture most things in the foundation, even though it wasn't known as the foundation back then, was prefixed NX, for NeXT, and sometime just before Sun entered the picture everything was renamed to NS. The S most likely did not stand for Sun then but after Sun stepped in the general consensus was that it stood for Sun to honor their involvement.
I actually had a reference for this but I can't find it right now. I will update the post if/when I find it again.