Considering the amount of philosophy written in Latin, I know there is a commonly accepted way of saying free will. I’m using libertas animi at the moment but I just put that together because it seemed to make sense.
edit: I wouldn’t normally make two threads in such a short period of time, sorry
Also, knowing how Medieval Latin occasionally employs different words, I’d like to know if there was a term that was more used then, as it would fit with what I’m writing.
For Church Latin, i.e., discussions of the free will by St. Augustine, St. Thomas, etc., the phrase used comes right from St. Augustine’s work De Gratia et Libero Arbitrio. But I would just take it as liberum arbitrium. As the notion would stem directly from St. Paul’s epistle to the Romans, you may wish to check the Vulgate to see what phrase St. Jerome used. (Oh When the Saints Come Marching In…)
As for a classical usage, I would defer to others here but I’m guessing it’s going to be close to that…simply that one acts arbitrio suo.
I’d triple check this if the dogs didn’t need an immediate walk. 
Hope this helps,
Chris
I think Ovid inter alios uses ‘sua sponte’ in some poems, but I’m not sure whether this is any use for a philosophical discussion on free will.
It works perfectly, thanks.
Augustine even has a writing titled “Concerning Free Will”: De libero arbitrio (edit: just realized the first reply pointed to this writing…though I think the title is this shorter one.)
A search through the Brepols Latin database reveals that liberum arbitrium was used 419 times before 500 C.E. That phrase is not in the Vulgate. In the Medieval period (735-1500) the phrase comes up 1103 times.
Because it’s not Vulgar Latin (which you probably know). I’m studying Paden’s book on Old Occitan, and I was nicely surprised to find that the Latin language has been rather more extensively defined than when I first started studying it. Paden uses these distinctions between the types of Latin extant at the time of the troubadours :
CL = Classical Latin
VL = Vulgar Latin
ML = Medieval Latin
ChL = Church Latin
LL = Late Latin
To which I would add OL for Old Latin, but that isn’t germane to Paden’s work.
I love it, it all indicates to me that Latin studies are alive and kicking in the 21st century.
Well, the Vulgate is sort of an exception to the typical Classical or Medieval Latin work…remember, it is a translation first and foremost…it is also a translation purposely not written in the prosaic style of authors like Cicero but rather in one more easily understood. It certainly should not be lumped in with all works before 500 C.E. for any statistical analysis without many, many footnotes. 
St. Augustine is expounding upon the Pauline epistle to the Romans in his work De Gratia et Libero Arbitrio. (N.B. Though you may see this tranlated as On Free Will that should not suggest to you that there are two books of St. Augustine’s on this subject.) The Pauline verses under discussion (Romans 6:16-23) are beautifully poetic and are not so rough as to simply blab out the phrase “liberum arbitrium.” You can read them here at the Latin Library. This is chapter where we find stipendia enim peccati mors, or, “For the wages of sin is death.”
St. Paul speaks metaphorically of free will in terms of obedience (a nice juxtaposition). Free will is whether one chooses to be obedient to sin or obedient to righteousness. The language St. Jerome uses is thus couched in this metaphor and you won’t find liberum arbitrium in or near the passage. That said, it is St. Augustine who makes that particular phrase significant for the concept poeticized by St. Paul…and is quite possibly why you find this particular phrase used so much more often in the documents that your Brepolis search uncovered during the Middle Ages.
Chris
oh yeah…of course. “Free will” is not a technical term in the bible…it is a theological term developed later (which is not to say that Augustine’s alternative is anymore “biblical” either…)