It's not really a word, rather a phrase, but has a meaning distinct from its relative "de facto" which means existing without legal authority. I presume is it Latin. Pro facto is literally translated as "for the fact", but it rather means considering or assuming a stated proposition as if it were fact. As if. That is to say, in doing so you realize that there is uncertainty, but you go with the best explanation until there is a challenge. Ruiz would suggest that we ought to avoid assumptions, and just admit to not knowing. But the world is much easier to manage when you have a framework for it.
What provoked me to look this up is the fact that the organization known as Oregonians for Science and Reason has a newletter by that name. What exactly did they mean whean choosing that title? That they were admitting that we are going with a working understanding of things that is subject to challenge, perhaps?
Please correct me if I have the shades of meaning wrong. Gracias.