A closed path through spacetime that is timelike. A CTC would correspond to a possible way of time travel or time communication. However no closed timelike curves have ever been observed to exist.
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Chronology Protection - Text by Anders Sandberg The universe seems to counteract attempts to produce time machines, for example due to Visser decay and related vacuum phenomena undermining other forms of time communication. Although the strict Chronology Protection Conjecture by the information age physicist Hawking was subsequently disproved, the chronology protection theorems of Lang, Picard and Joel-5 showed that this tendency can be explained from the consistency theorems, information physics and (eventually) vacuum physics.