You want to define the NU tier? Okay.
The NU tier is a list of all Pokemon that are not in the Uber, OU, BL and UU tiers. If a ban list for NU is created, then NU would not have the Pokemon in this ban list as well.
So really, the question is not how to define the NU tier, but how to define all the other tiers. NU would then have the simplest definition of all: it'll be the remaining Pokemon.
OU and UU would be defined in the same way as per usage statistics. Their size is the only subjective thing about them (the "how rare is rare?" problem), but, other than that, they're completely objective tiers.
What aren't settled are the Uber tier, BL tier and, if it's going to exist, the ban list for NU. These take much longer to settle as they're not mostly objective, like OU and UU, but mostly subjective. I've been researching for a long time to try to objectify these tiers as much as possible, but I'll be the first person to state (with objective proof) that this is impossible to do completely. The Base Stats Ratings and the average damage that all the types take are a start in this direction, but, in the end, someone must take a definitive stance at some point and say "these Pokemon are Uber/BL/(banned from NU), and you must accept these tiers no questions asked". Yes, either that will happen, or else these tiers will never be stable.