Super warm down jacket.
Update: This thing is crazy hot. I put it on to go for a walk in 50F weather and within five minutes I was sweating. There is absolutely no reason to wear this above 40F, maybe even 30F.
In retrospect, I would have been better off with the lighter version of this to wear over my Nano-Air, but oh well. Now I just have to swap them as I get warmer or colder.