Here's my definition of "free will".
Let say there's a decision tree where each node is a decision, and its children are the outcomes of each decision. Let suppose the decision tree only have nodes that are reachable from an individual (for example, if a choice kills you, you can't make any more choices after this point).
The "will" of a person is the heuristic function he or she uses to pick the next best choice according to said function.
"free will" is then to always have the freedom to change this heuristic function to whichever one you want, so that nothing impedes you to reach any node of the decision tree. For any node, such heuristic function exists and it's "pick the choice that reachs this node".
So all discussions about "free will" is about if there's a barrier of the subconscious that partially disallows a person to change his will, for example, the so-called "survival instinct".
It's known that many animals have "hard-wired" behavior and react to events by "instinct", and as such are not considered to have free will.
It's known that man works by instinct at certain situations, and that his instinct can be overcome by training. For example, that CIA training to avoid turning around when you are touched in your back.
So in some sense, "free will" can be achieved in humans by training. It also can be temporarily lost by chemical/electrical impulses such as drugs or body secretions.
EDIT: It looks like SMBC just drew a comic about this. Coincidence, or a chemical shift? :P
http://www.smbc-comics.com/index.php?db=comics&id=2358#comic