A "주기 (Cycle)" refers to a series of events that repeat in a predictable pattern over time. This concept is commonly observed in nature, such as the water cycle, where water evaporates, condenses, and precipitates, continuously cycling through different states. Cycles can also be found in various fields, including economics, biology, and astronomy. In biology, for example, the cell cycle describes the stages a cell goes through to grow and divide. Similarly, in astronomy, the lunar cycle tracks the phases of the moon as it orbits the Earth. Understanding cycles helps us predict and analyze patterns in different systems.