Determinism is the philosophical proposition that every event, including human cognition, decision and action, is causally determined by an unbroken chain of prior occurrences. Thus, there is at any instant only one physically possible future, and no random, spontaneous, mysterious or miraculous events ever occur.
This posits that there is no such thing as Free Will, where rational agents can exercise control over their own actions and decisions. Incompatibilists (or Hard Determinists) like Baruch Spinoza, view determinism and free will as mutually exclusive. Others, labeled Compatibilists (or Soft Determinists), like Thomas Hobbes, believe that the two ideas can be coherently reconciled.
It should be noted that Determinism does not necessarily mean that humanity or individual humans have no influence on the future (that is known as Fatalism), just that the level to which human beings have influence over their future is itself dependent on present and past.