Determinism is the view that every event, including every human decision, is the inevitable result of prior causes stretching back through an unbroken chain — which seems to leave no room for choices to have gone differently than they did. Free will seems to require exactly that: the real possibility of having chosen otherwise. Compatibilism is the position that these two ideas aren't actually in conflict, because the traditional argument against them being compatible rests on a definition of "free" that isn't the one that actually matters.
Two different questions hiding inside "could you have done otherwise?"
The classic argument against free will runs: if determinism is true, every choice you make was fixed by prior causes before you were even born, so you never really "could have" chosen differently — and if you couldn't have chosen differently, you weren't free. Compatibilists challenge the second step, arguing there are two very different things "free" could mean. One is metaphysical: could the exact same universe, with the exact same prior state, have produced a different choice? The other is practical: was your choice the product of your own reasoning, desires, and deliberation, rather than being externally coerced, compelled by force, or the result of manipulation or a mental disorder overriding your judgment? Compatibilists argue the second kind of freedom is the one that actually matters for holding people responsible for their actions — and that kind of freedom is entirely compatible with determinism, because it's about the source of the choice, not about whether the universe's prior state could have unfolded differently.
Why this isn't just wordplay
The compatibilist move can sound like it's dodging the hard question by redefining "free" until the problem disappears, and incompatibilists — philosophers who think real freedom requires the genuine metaphysical possibility of having done otherwise — make exactly that objection. Compatibilists respond that this is how the concept of freedom actually functions in ordinary life and law: when courts distinguish between someone who commits a crime under coercion and someone who commits the same crime by their own deliberate choice, the distinction being tracked isn't whether the universe could have unfolded differently in either case — determinism, if true, applies equally to both. It's whether the action flowed from the person's own will, unforced and unimpaired. Compatibilists argue that's the concept of freedom doing real, meaningful work, whether or not the deeper metaphysical question about the universe's causal structure ever gets resolved.
What we're still unsure about
Compatibilism is one major position among several live options in this debate, not a settled consensus — libertarian free will (the view that genuine, undetermined choice is real and requires indeterminism somewhere in the causal chain) and hard determinism (the view that free will, properly understood, simply doesn't exist) both remain seriously defended positions in contemporary philosophy. The debate also increasingly draws on findings from physics and neuroscience — whether the universe is even fully deterministic at the quantum level, and what brain-imaging studies purporting to detect decisions before conscious awareness actually show — without either field having settled the philosophical question, since even a fully deterministic or a partly random universe leaves the compatibilist's practical definition of freedom, and the objections to it, exactly where they were.
This sits inside Free Will & Determinism, one of seven topics in Metaphysics, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.