A good question.
The defining property of free will, i.e. freedom in choices, is the "feeling" or consciousness of the own authorship of the choice being made. Even if/when I'm forced to comply I cannot escape the glimpse, at the moment, that nothing actually is selecting the way for me, except that "foolish" me (who is, actually, nobody/anonymous this minute) who agrees with the offered.
Thus, every human choice is a free choice, and responsibility is implied automatically. That would be Sartre's account.
Moreover, even when I stumble and fall down, I immediately and promptly react and interpret the occasion (e.g. by the fear to break an arm and not be able to paint anymore through that). By application of a free interpretation/meaning I emerge responsible for my incident though I didn't choose to stumble. I selected to be thus fallen, instead.
Freedom is spontaneous choice (of meaning and action) accompanied by the awareness of own authorship / resposibility. Or to put it differently, the apprehension that there is nobody/nothing specific is to blame except myself.
The key point here, @HWalters, is that ownership belongs to (is felt by) faceless me, the Nothing, and not to I, the Something. If nothing is the decision maker, the decision is, logically, not motivated by any preexisting structure, i.e. it is not determined (subjectively) by any cause. It is motivated by no cause, a pure gap in causational chain. (It is but later that we can attribute a cause to our decision, to "excuse" it.)
Thus, freedom is human (i.e. conscious) randomness (conditioned upon given circumstances). Within the scope of possible alternatives the circumstance can support, it is non-determined.
A man is an entity whereby randomness becomes own while determinism gets broken. Whenever consciousness comes to play, whenever something comes to being through consciousness, determinism is canceled.
Even conceived/planned decision without instant action is not your "will", it is spontaneous. It was spontaneous when it first occured to you, and it is again spontaneous when you start to act according to it: you re-invent it one more time, using your knowledge of the plan as the circumstance, and act.
So, free will is freedom. Specifically "will" is a redundant word, and people do not do things by "will" as some tension or effort. It is when I reflect back on the thing done or on the difficulty to overcome to do it, I might see that "labour" of my Ego involved and start to think of (free) "will" as my instrument. "Will" is always an artefact of reflection ("I decided and did it myself", "I must decide to do it"). No "will" is actually used in real, here-and-now decision makings.
Also to say: curiously, non-human (out there, universal) randomness is close, in eyes of a human, to determinism (or doom) and not to indeterminism. Both random and predetermined appear to us as contingency which is a characteristic of facticity to where we are "abandoned". Marooned for making free choices in the flight from facticity towards meaningful world.
P.S. Once again, how I feel it, and in a response to a comment elsewhere, where they've just asked me how freedom is different from randomness:
Freedom (free will) is spontaneous self-determination. Randomness is when the cause, "author", cannot be indicated, there is none such. Whereas here we have the cause - it is the prereflective, nonthetical self.
That self, being the cause of a choice, is also causa sui; it is not a consequence of another, so it is not a mediator for the choice, - that is because it is non-being: it is not based/founded on being, on what is.
Further, it (the self) is not a tuple pair (self & choice), but one monad: the self selects itself as one possibility of the object-in-the-world (this glass of water in front, which is so drinkable) and emerges in this act. The self thus generates itself and generates its choice in a single pass, one event. It does not pre-exist this functional act of choosing as something, but coincides with it as nothing. (Nothing is shortage of..., unclasping of..., not an emptiness.) I'm choosing myself as the possibility (or potentiality, or opportunity - don't know for now which word is better) of an object, me-possibility existing as deferred here, but strictly inside the circle of conditions (givennes, including my own empirical Ego) which I therefore appear beyond.
So, the causality with consciousness, or free will, is special: via self-nothingness rather than via being/matter. Spontaneity, aka free self-determinism.