1. n. The basic unit of computation. What every hacker wants more of (noted hacker Bill Gosper described himself as a “cycle junkie”). One can describe an instruction as taking so many clock cycles. Often the computer can access its memory once on every clock cycle, and so one speaks also of memory cycles. These are technical meanings of cycle. The jargon meaning comes from the observation that there are only so many cycles per second, and when you are sharing a computer the cycles get divided up among the users. The more cycles the computer spends working on your program rather than someone else's, the faster your program will run. That's why every hacker wants more cycles: so he can spend less time waiting for the computer to respond.
2. By extension, a notional unit of human thought power, emphasizing that lots of things compete for the typical hacker's think time. “I refused to get involved with the Rubik's Cube back when it was big. Knew I'd burn too many cycles on it if I let myself.”
3. vt. Syn. bounce (sense 4), from the phrase ‘cycle power’. “Cycle the machine again, that serial port's still hung.”