With regard to philosophy, determinism is the idea that everything that happens has happened due to a set of circumstances that made it inevitable. In other words, every time that a given set of circumstances comes to exist, there is a certain outcome that is guaranteed to happen as a matter of natural law. If only we understood the causes and effects well enough, we might be able to predict what would happen in any given circumstance.
Although this idea has been around for a very long time, it came to be more popular and more thoroughly studied and discussed in the Enlightenment. This makes sense because this idea is in some ways very scientific. It holds that the universe is governed by natural laws and that, at least theoretically, we can understand those laws and use them to predict what will happen in the world.
The idea of determinism is often seen as being in opposition to the idea that human beings have free will. If our actions are determined by sets of circumstances, then it is arguably impossible to say that we are free agents.