As you may have noticed, above you will find words for "A light-like boundary of the domain of validity of a Cauchy problem. One side of the horizon contains closed space-like geodesics and the other side contains closed time-like geodesics.". Hover the mouse over the word you'd like to know more about to view its definition. Click to find a better fitting word. Finally, thanks to ChatGPT, the overall results have been greatly improved.