In a nutshell, as I understand it, it would be the ability of the individual to to choose to follow or reject God; to obey or disobey at the discretion of the individual, without being forced or compelled to.
I’m sure that some will find that lacking in some area, and there are probably things I could add, given more thought, but that’s basically it.
Oh. Well, if that's your definition of Free Will, Calvinists don't deny that.
Rather, Calvinists state that since the Fall, Unregenerate Men (which is all of us, prior to God's regeneration of our dead spirits) freely choose to Reject God, because that is what Unregenerate, Spiritually-dead Men want to do.
What would be heretical would be the suggestion that any Unregenerate Man would ever freely choose to Follow God while yet in their Spiritually-Dead condition; because the Bible clearly states that while a man is yet Spiritually-Dead his desires are Totally Depraved:
And so, while yet in his Spiritually-Dead state, the Unregenerate Man will always freely choose to Reject God. This is the express teaching of the Bible:
To suggest, therefore, that Men will freely choose to Follow God while yet in their Unregenerate State, is to preach the lie of Satan: that Fallen Men are not really Spiritually Dead, and will still sometimes freely choose to perform God-Pleasing actions. "Ye shall not surely die".
Thus, we as Calvinists understand that Men only freely choose to Follow God, after they have been Quickened unto Spiritual Life by the Efficacious Grace of God.
Calvinists do not deny Free Will.
Rather, we simply affirm that Fallen Man REALLY IS Spiritually Dead. And that while a Fallen Man remains Unregenerate, he will always freely choose to Reject God, because that is what he Naturally wants to do.
Thus, only those whom God chooses to Regenerate unto Spiritual Life, do thence freely choose to Follow Him.