While there is an agency that defines water resistance, if Nikon is testing to those standards they don't disclose or market using them.
- IPX0 — no special protection.
- IPX1 — Protected against falling water equivalent to 3-5mm rainfall per minute for a duration of 10 minutes. Unit in normal operating position.
- IPX2 — Protected against falling water when tilted up to 15 degrees (in each of four directions).
- IPX3 — Protected against spraying water 60° from vertical at 10 liters/min and a pressure of 80-100kN/m2 for five minutes.
- IPX7 — Protected against water immersion for 30 minutes at a depth of 1 meter.
- IPX8 — Protected against continual submersion, manufacturer must disclose conditions fully.
I've left out a few levels, as they don't really tend to apply to photography (you couldn't keep a lens clean at IPX4 through IPX6 levels, in my opinion, so you wouldn't be using a camera in those situations).
By not disclosing a water resistance, Nikon is essentially claiming IPX0: no protection. In reality, most of the pro gear (D300s, D810, D4s) certainly seems to hold up to IPX1 standards, and many of us have used them in IPX2 levels without consequences. But that's not guaranteed. Thus, you have to balance the risk of having real problems with your camera versus getting the picture in light mist and rain situations.
Fortunately, there are plenty of simple solutions, ranging from just tying a baggie over your camera to dedicated and complex rain covers. In serious rain, I tend to use ThinkTank and/or AquaTech covers for my gear.
A few camera makers do claim "splash proof" for their cameras, but it usually turns out to be a rating of IPX1.
Short answer: if you want a camera guaranteed to be usable in very wet conditions, you need something like a Coolpix AW-110, a Nikon 1 AW1, or Olympus Tough TG-3 that is rated IPX8, or a housing for your camera that has the same level of rating.