Via Cult of Mac:

We’ve all noticed Apple’s latest operating system nomenclature, with each new release a device-centric OS, like iOS, tvOS, or watchOS. Why not macOS, then?

Likely what we’ll see at WWDC later this year. To be honest, I’m surprised they didn’t make a change like this sooner if only to avoid the weird 10.10.X / 10.11.X numbering of OS X these past couple years.