Time

From Memory Alpha, the free Star Trek reference.

"Someone once told me that 'time' is a predator that stalks us all our lives. But I rather believe that time is a companion who goes with us on the journey and reminds us to cherish every moment because it will never come again."

- Jean-Luc Picard (Star Trek Generations)

Time is a nonspatial continuum in which events occur in seemingly irreversible transition of the current state to the next as governed by the laws of the local universe.

Physicists often describe the universe as having three spatial axes: x, y, and z (though the names often differ according to local customs). Time is often regarded as the fourth dimension; however, for the most part, time progresses on its own in one direction.

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An important consequence of the perception of time as another dimension is that traveling backwards in time is possible. Just as one can change direction in space (say, go left instead of right) one is able to traverse time in a similar manner.

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At this time, the Federation has very limited experience in the controlled traversal of time. Most of the time travel was either accidental or caused by the intervention of a technologically superior species. Current Federation doctrine, namely the Temporal Prime Directive, prohibits using time travel to alter the past.

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While Star Trek has portrayed that time could be made to flow backwards due to certain phenomenon such as distortion fields (as in TNG: "Timescape"), current knowledge of physics does not allow for that interpretation of time.