Travels in time
In the descriptions, the time machine usually appears as apparatus, than moves human(s) to the past or to the future.
Such a movement is interpreted as "travel in time", although it may have also other meaning (See, for example, Back in the USSR).
In classical mechanics, the travel in time, and, in general, any time machine causes paradoxes.
In quantum mechanics, hypothesis of existence of a time machine does not cause paradoxes, although, the basic concept of causality applies only at the small scale, for the time intervals, that are shorter than time distance, at which the time machine woks.
At the quantum consideration of time machine, the trajectories, that correspond to the classical paradoxes, appear with zero amplitude, and, therefore, do not affect on calculation of the observable quantities.