Indeterminism simply means that not all events that happen have a cause. Some events simply happen without anything leading to them. Those events are sometimes called uncaused, acausal, non-caused, or not caused events. The term I prefer is “acausal”, but any of these will do the trick.
I don’t like to use certain terms such as random or spontaneous unless they are absolutely necessary. Such terms have ambiguous meanings, some of which don’t mean the same thing as being “acausal”. For example, some might consider a roll of dice to be “random” simply because we don’t know what the outcome will be. Such, however, isn’t truly random in the acausal sense. Technically the dice being thrown a specific way, the air, the gravity, the ground it lands on, the weight of the dice themselves, and so on… are all causal factors that output the results of the dice. If you’ve read the article on determinism you’d see that determinism doesn’t imply predictability. The use of the word “random” in the sense of dice being rolled can happen in an entirely deterministic (causal) universe. A word like “acausal”, however, is clear. There is no confusing such a word. If an event is acausal, there is no cause for the event.
If we were to say “The Universe is Indeterministic”, it would mean that at least some of the events that happen in the universe just happen with no other existing factor forcing them. Indeterminism does not mean all events are like this – another common confusion people make. A mix of causal events and acausal events would be considered “indeterministic”. In fact, if there are just one or two acausal events in the universe, such possibilities means that the universe is “indeterministic”. It simply means that the universe is not entirely causal.
It’s important to note that we simply don’t know if the universe is deterministic or indeterministic. Some people who quote Quantum Mechanics make the claim that such proves the universe is indeterministic. They are wrong. It all depends on the Quantum Interpretation the person is subscribing to. Some interpretations, such as the Copenhagen Interpretation, are indeterministic, others, such as Bohmian Mechanics or a Many Worlds Interpretation are deterministic. There are a whole bunch of interpretations. The important thing to note about each of the interpretations is that they are not the science of Quantum Mechanics (though some such as the ensemble interpretation tend to make fewer assumptions) . They are “interpretations” OF the science. Think of them more as the philosophy of why things are the way they are within the mathematics and experiments of Quantum Mechanics.
Each of the interpretations have parts that are very counter-intuitive. An indeterministic interpretation that assumes acausal events happen, have to contend with what it means to say that an event has no cause. I go much deeper into this within my book, but there are problems that need to be addressed when an event simply cannot have any spacial or temporal determinacy (which means it could happen anywhere, at anytime, or never). That’s not to say that a deterministic universe wouldn’t have troubling counter-intuitiveness built in as well (e.g. non-local hidden variables, decoherence, etc.).
What’s important, and what I stress, is that even if such an indeterministic universe existed, any acausal events that occur could not help free will. They would be every bit incompatible, and in fact, it is more likely to make things much worse to any creature in which an acausal event happens for the process of a decision. At best, if the acausal event just happens to produces the same results a causal event would, such would be benign.
If you want to learn more about the nature of acuasal events in an indeterministic universe, I go into greater detail within Breaking the Free Will Illusion for the Betterment of Humankind.
For further information on Indeterminism and some of the confusions that arise with this term please read: