Ozgeorge is absolutely right...as always. However, if by free will you mean man's freedom to either turn towards good, or bad, then there's one Patristic word describing that:
Αὐτεξούσιον (Aftexoosion, a compound word-->αὐτο-, a prefix relating to one's self, and ἐξουσία, to govern or to rule) which, according to st Cyril of Jerusalem, is man's freedom of the will.
According to st John of Damascus, the main and cardinal attribute of our nature is the αὐτεξούσιον:
"Πάσῃ γὰρ λογικῇ φύσει ἐμπέφυκε [ὁ Θεός] τὸ αὐτεξούσιον θέλημα-aftexoosion thelima"
"[God] in every rational natural has implanted free will".
If you want a more "modern" philosophical approach, then ἐλευθέρα βούλησις (elefthera voolisis) in Katharevousa is correct or ελεύθερη βούληση (eleftheri voolisi) in modern, colloquial greek. Both of course, are a direct translation of the latin libera voluntas.
Note: The bold letters suggest the stress
[sorry I had to edit my post]