Cycle

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

Romulan cycle
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 (NCC-1701) had been motionless for 9 hours and 47 minutes. 

Argrathi cycle
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 implanted memory). 

Borg
Borg seemed to count time in cycles. For example, assimilated children are put in a maturation chamber for 17 cycles. 

Other uses

 * Cycles seemed 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