Determinism: The notion that what
happens was bound to happen.
Classical physics was
deterministic: Given such and such a physical situation, in which so
and so happened, this and that must happen, caused by the
situation and the event, "because of the nature of things".
This changed with the arisal of quantum mechanics, where what
happened on a sub-atomic level is supposed to be non-deterministic and
essentially unpredictable in precise and determinate detail.
The problems and confusions about determinism are not so much
physical as moral: Both determinism and
indeterminism, as these words are used in physics, seem incompatible
with free will such as most persons
believe this holds for them, and such as is used in the law to impute
responsibility to persons for their actions.
The main reason is that most persons - including judges and
prosecutors in courts - will insist and believe that many human actions
were willed by the persons who did them; that they were free
to will other than they in fact did, and in this sense not
determined by anything other than their own choice, which was freely up
to them, only dependent on but not determined by their preferences and
beliefs; and that also what they did choose in fact was not a mere matter of
chance, but deliberately chosen, because the alternative seemed worse,
yet not necessarily so, since even if one strongly prefers an
alternative one has the freedom to perversely choose something else.
Also, those who believe that physics - apart from quantum-mechanics - is deterministic often miss the real reason for this, which is not some insight about the nature of reality or causation, but the fairly simple mathematical fact that what got used in most physical theories are functions, in the mathematical sense, which lead from given inputs to one single output.