Determinism is the concept that events are caused by prior events. Any state (of an object or event) is determined by prior states.
Determinism supposes that every event, including the individual actions of every human is causally determined by previous events. Determinism is often contrasted with the concept of free will.
Determinists believe the universe is fully governed by causal laws resulting in only one possible state at any point in time. Determinism proposes there is an unbroken chain of prior occurrences back to the origin of the universe, hence given the initial state of the Universe, the present state is the only possible state.
The determinist view is consistent with Newtonian mechanics, but not quantum mechanics, which treats events on a quantum scale in a probabilistic manner, and says for exam that each atom in a radioactive sample of identical atoms is equally likely to decay. If any atom can decay, more than one outcome is possible, obviously in contradiction with determinism.