A cycle was a unit of time used by several species.

Argrathi cycle Edit

An Argrathi cycle was equivalent to one Earth year. The crime of espionage required a minimum of fifteen cycles of correction on Argratha, and Miles O'Brien served 20 cycles, the equivalent of twenty Earth years (though his entire imprisonment was actually an artificial memory). (DS9: "Hard Time")

Borg cycle Edit

The Borg seemed to count time in cycles. For example, assimilated children were put in a maturation chamber for seventeen cycles. (VOY: "Mortal Coil")

Seven of Nine spent five years in a maturation chamber, so one cycle could conceivably equate to approximately 107.4 days.

Ferengi trading cycle Edit

A Ferengi trading cycle was a period of time used to measure time on commodity markets, such as duranium prices on the Futures Exchange. (DS9: "Ferengi Love Songs")

Romulan cycle Edit

One Romulan cycle was approximately equivalent to no more than 29.35 Earth minutes. A Romulan officer noted they had no motion on their sensors for twenty cycles. James T. Kirk had also noted in a log that the USS Enterprise had been motionless for nine hours and 47 minutes. (TOS: "Balance of Terror")

In the background chatter of TNG: "Unification II", it is possible to hear dialogue noting something about "two cycles". The exact line is "We are very close now... within two more cycles."

Other uses Edit

A cut scene from "The Inner Light" mentions the Ring cycle.
The use of the word cycle as a "generic" alien unit of time is common in science fiction, to the point that it is almost a cliché. No doubt this was a factor to its many uses in the Star Trek universe.