There are 3 aspects of "free will": religious, ethical and scientific. In the scientific case "free will" implies that the actions of the body, including the brain and the mind, are not wholly determined by physical causality.
UPDATE: There are cases where a patient experiences loss of brain function leading to an inability to make decisions. They require a guardian to make even the most trivial choices for them (e.g. what to wear, in what order to eat the items in their meal, etc.) What exactly have they lost? An answer to this question, scientific or otherwise, would be relevant to free will.