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

Romulan cycle

One Romulan cycle is 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 9 hours and 47 minutes. (TOS: "Balance of Terror")

Argrathi cycle

An Argrathi cycle is 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 implanted memory). (DS9: "Hard Time")

Borg

Borg seem to count time in cycles. For example, assimilated children are put in a maturation chamber for 17 cycles. (VOY: "Mortal Coil")

Other uses

• Cycles seem to have a strong role in the religion of Borothan pilgrims. Amongst their religious concepts were the Cycle of renewal, the Cycle of creation and the idea that the universe goes through cycles of rebirth.
• REM cycle
• the life cycle