These are tricky waters, but here's what I see as the difference: Two separate threads that can run completely independently without any synchronisation are co

These are tricky waters, but here's what I see as the difference: Two separate threads that can run completely independently without any synchronisation are concurrent because it doesn't matter what order you run them in. Therefore there are many, many 'sequentially equivalent' computations that are correct per the language semantics. From the perspective of the user it can seem like you ran Thread A to completion followed by Thread B, or Thread B then Thread A, or any interleaving of the two. Threads are one way of expressing concurrency. Actors, to pick at random, are another. So I don't believe that threads <=> concurrency, but that threads usually express concurrency unless they have pathological synchronisation behaviour. Which they would have, in your final example - there is no concurrency because there's only a single ordering of events that is correct.