The doctrine that every event has a cause. The main philosophical interest of determinism has been in assessing its implications for free will.
The doctrine that every event's causation can be understood wholly in terms of the consequences of past events or the deterministic will of an all powerful being.
One common version states that for every event, there is an antecedent state, related in such a way that it would break a law of nature for the antecedent state to exist yet the event not to happen.
This is a purely metaphysical claim, and carries no implications for whether we can in principle predict the event - e.g., chaos (i.e., it is not an epistemic claim).
The main philosophical interest of determinism has been in assessing its implications for free will. Thus, it is also often an integral question in philosophy of mind (especially in terms of the relationship of mind and body) and philosophy of religion.