For the purposes of this market, a "nuclear ultimatum" will be if an official Russian government source issues a threat of a nuclear strike connected with any specific action on the part of another actor. 

"Calling their bluff," in this case, would mean that the US or another NATO member chooses to take that action (or support it if the threatened party isn't a NATO member). Again, statements by official state sources will be determinative. 

I will set a deadline for the parties to do the thing (or not) by the end of March 2023. The market closes at the end of Feb 2023 - if no ultimatum is issued by that date, I will resolve the market as N/A. At the end of March, if the ultimatum has been issued but NATO/the US has not yet violated it, I will resolve this as NO. I feel the YES resolution will likely be obvious if it occurs. 

Oct 7, 10:49am:  If it's relevant, I might choose to bet in this market, though I don't presently have a strong prior either way. 