There are many terms from classic and modern SF that remain unresearched, and the Historical Dictionary of Science Fiction will be continually updated, especially as additional resources are put online. Boing Boing is syndicating new entries from the HDSF on a regular basis. (Read the series introduction.)
Perhaps the most classic trope of time-travel stories is that of the time paradox (of which the grandfather paradox is itself the most classic subvariant): What happens if you go back in time and change something that affects the future from which you left? Science-fiction writers have come up with numerous ways of addressing this, but whatever mechanism is invoked to explain it, it usually remains true that changing the past is a big potential problem.
Thus, the existence of a time police, a governmental body that regulates time travel, to prevent what is usually referred to as violations of "causality," or changing the cause-and-effect model underpinning the idea of time paradoxes. And someone who attempts such a violation is, clearly, guilty of a time crime.
This term first shows up in the 1950s, used as a story title by H. Beam Piper, whose Paratime sequence presented a parallel-universe version of time travel, featuring a Paratime Police who can move between different timelines, some more advanced than others. As a term, it is less common than time police, perhaps because these officers are, thankfully, good at their jobs.