deterministic describes a system, process, or rule whose outcome is fixed entirely by its initial conditions and inputs, so that the same starting point always produces the same result and nothing is left to chance.
The word turns on determinism, the principle that every event follows necessarily from prior causes. In mathematics and computing a deterministic function returns the same output for any given input every time, and a deterministic algorithm moves through the same sequence of states on the same data — behavior that makes such systems predictable, testable, and reproducible. In philosophy the adjective carries the heavier claim that human choices too are fixed by antecedent conditions.
The term is sharpest against its opposite. A nondeterministic, stochastic, or probabilistic process folds in randomness, so identical inputs can yield different outcomes on different runs. Determinism is therefore less a property of a thing in isolation than a statement about the relationship between its inputs and its outputs: fix the inputs, and a deterministic system leaves nothing else free to vary.