General:
* Cycle: A complete round or series of events.
* Cyclops: A giant with a single eye in the center of his forehead.
* Encyclical: A circular letter, especially from a pope or other religious leader.
* Cyclic: Relating to or involving a cycle.
* Cyclical: Recurring or happening in cycles.
* Cyclotron: A type of particle accelerator.
* Cycloid: A curve traced out by a point on a circle as it rolls along a straight line.
More Specific:
* Autocycle: A motorcycle.
* Bicycle: A vehicle with two wheels that is powered by pedals.
* Tricycle: A vehicle with three wheels.
* Unicycle: A vehicle with one wheel.
* Cyclometer: A device for measuring distance traveled.
Scientific terms:
* Cyclonic: Relating to a cyclone.
* Cyclone: A system of winds rotating around a center of low atmospheric pressure.
* Cyclohexane: A colorless liquid hydrocarbon.
* Cyclopentane: A colorless, flammable, volatile liquid hydrocarbon.
This is not an exhaustive list, but it gives you a good overview of the many words that come from the Greek root "kyklos."