An acausal event is one which [i]actually[/i] has no cause, not just one where it [i]appears[/i] to have no cause. A causal loop wouldn't be considered acausal since, to use the explanation I gave previously, a causal loop involves event A causing event B, followed by event B causing event A, which then causes event B, and so on, and therefore still involves one event causing another. In the case of event B causing event A, it may appear that event A has no cause since the temporal order of causation has been reversed, in the sense that event B occurs after event A, but this does not mean that there is no event which causes event A. It can be difficult, if not impossible, to determine which event, A or B, was the origin of the causal loop but it does not follow from this that there is no event that was the cause of the causal loop, or that the events within the causal loop do not themselves have causes. You are correct though in that causal loops are also known as ontological or bootstrap paradoxes. Another very well-written story by Robert Heinlein involving causal loops is "By His Bootstraps".