A basic unit of computation, one period of a computer clock.
Each instruction takes a number of clock cycles. Often the computer can access its memory once on every clock cycle, and so one speaks also of "memory cycles".
Every hacker wants more cycles (noted hacker Bill Gosper describes himself as a "cycle junkie"). 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.
The use of the term "cycle" for a computer clock period can probably be traced back to the rotation of a generator generating alternating current though computers generally use a clock signal which is more like a square wave. Interestingly, the earliest mechanical calculators, e.g. Babbage's Difference Engine, really did have parts which rotated in true cycles.