Determinism is a philosophical view that states that events are completely determined by previously existing causes. It is the idea that every event is necessitated by antecedent events and conditions together with the laws of nature. Deterministic theories have developed from diverse and sometimes overlapping motives and considerations throughout the history of philosophy. The opposite of determinism is indeterminism, which is the view that events are not deterministically caused but rather occur due to chance.

Determinism is often contrasted with free will, which is the supposed power or capacity of humans to make decisions or perform actions independently of any prior event or state of the universe. Philosophers and scientists who deny the existence of free will on the basis of determinism are known as "hard" determinists. In contrast, "soft" determinists, also called compatibilists, believe that determinism and free will are compatible after all.

Determinism is deeply connected with our understanding of the physical sciences and their explanatory ambitions, on the one hand, and with our views about human free action on the other. There is no agreement over whether determinism is true or false, and what the import for human agency would be in either case.

In summary, determinism is a philosophical view that events are completely determined by previously existing causes, and it is often contrasted with free will.