Most developers mistakenly think NS stands for NeXTSTEP. (NSApplication, NSArray, NSMutableDictionary, etc).
NS actually stands for NeXT-Sun. It came about when NeXT partnered with Sun Microsystems to develop OpenStep, which was an API specification for developing applications on non-NeXTSTEP operating systems.
Before the partnership, NeXT's APIs began with NX.