The laws of physics can be said to obey certain symmetries. Symmetry C is when the laws for particles are the same as their anti-particles, P is when the laws are the same in the mirror image of any situation, and T is when the laws remain the same if time were reversed.
Symmetry Term Timeline in A Brief History of Time
The timeline below shows where the term Symmetry appears in A Brief History of Time. The colored dots and icons indicate which themes are associated with that appearance.
...allows time travel on a miniscule scale. As particles follow the C, P, and T symmetries, a particle going backward in time could be considered an anti-particle going forward in time.... (full context)