For my money its not a boundary until theres a clear statement of the limits of behavior one will accept and an action that is taken if/when its crossed. I think it works both ways; my boundary is that if my wife resumes the old drinking behavior then either she leaves or I leave with our daughter and let the house and 401k be damned. That also means I cannot resume <my> old behavior of resentment and anger towards her. In effect, I am responsible for making some considerable changes in my own behavior if any statement about action in response to her behavior is to be consistent.
I've had to make other boundaries with my still-active mother, and am considering how to handle the gossipy mother-in-law. The leading prospect for the latter is to strictly avoid all conversations about my business or about 3rd parties who are not present. lol, not sure what that leaves to talk about though...