Imho to what generation the NX belongs will only be able to be said in retrospect. It remains to be seen what its main competition will be:
- if most NX units/games are sold after other "9th gen" consoles are released, then it's clearly a 9th gen console
- if most NX units/games are sold before the PS5 or XBox "Two" release it will be more complicated:
either you classify the NX as the sole 9th gen console, making the PS5 and Xbox "Two" 10th gen consoles; or you say that the NX is a 8.5th gen console (maybe together with the PS4K and Xbox 1.5); or you say that the NX also belongs to the 8th gen as it sold most units/games competing with other consoles from that time period. 

In the meantime I guess it's most useful to see it as a 8.5th gen console, even if that sounds slightly awkward.

Only history will decide of that. Not me or you or even Nintendo.

It's not something you can really define, especially now sincee it is also defined by when the successor of the NX will come out

I guess some can't count then. 

Just like the dreamcast when it was released early in a generation the NX if released this year or next year will be the start of the 9th generation, the ps4.5 however will not count towards the 9th but remain in the 8th generation.

This!

/thread

To all those people saying it's a simple question because it's defined by release:

In that case Sega started the 8th (or 7th?) generation with the Dreamcast. Meanwhile, everyone considers the Dreamcast a 6th gen console.

Again this thread? It is 100% a 8th gen console.
NX will compeat against 8th gen consoles (PS4 / Xbox One) so there is no way that NX will be a 9th gen console.

nx will compete against 8 and 9th generatiom consoles, when i checked the wikipedia the dreamcast isin 1998 is the same gen as the ps2 but was launched to compet with n64 and psx before the ps2 arrived 2 years later and gc and xbox even later, so the nx is 9th gen i think.

That makes no sense. If that was the case, then wii would have been a 6th gen console because it competed with the PS2. *Rolls eyes

The console gens overlap with eachother and each gen's consoles compete with the consecutive gen's consoles. 

N64 is 5th gen
Gamcube is 6th gen
Wii is 7th gen
Wii U is 8th gen
Nintendo's next home console is 9th gen

It's not complicated. It has nothing to do with power or what the device competes with what. If nintendo replaces the wii u with it's successor, that's the next generation of nintendo consoles. 

Not true. 
Wii competed with the PS3 and X360 much longer than it did with PS2. So it is a 7th gen console.