Closed timelike curve (abbreviated CTC) is physics jargon
solution of general relativity
that describes time travel
. The 'curve'
means a world line of somebody, 'timelike' means that the world line does
not exceed the speed of light, so it is a 'valid' world line, 'closed'
means that the world line is a loop, which means that it meets itself at
an earlier time.
These things were invented by Kurt Gödel after meeting Albert Einstein
in The Institute for Advanced Study. Mathematically these solutions are
agreed to be valid, but their physical significance is unclear. It may be
that general relativity is not valid in the conditions where CTC:s would
One example of conditions, where general relativity makes CTC:s possible,
is a _very_ dense, _very_ rapidly rotating, _very_ heavy, object.
According to Einstein's field equations, such an object could curve the
space-time around it so much that you could travel backwards in time
by moving around in a suitable manner.