Cycle

A cycle is any process or structure that returns to its starting state after a sequence of steps, forming a closed loop or repeating pattern. The word is used across disciplines to describe phenomena that recur over time or through stages, often with a characteristic duration or order.

Mechanical and physical cycles include the four-stroke cycle of internal combustion engines, which proceeds through intake, compression, power, and exhaust phases, and electrical cycles in alternating current systems, where current reverses direction at a fixed frequency. Thermodynamic cycles such as the Carnot cycle model the idealized steps by which heat engines transfer energy. In general, a cycle can refer to any sequence of changes that repeats.

Natural and biological cycles encompass recurring processes in nature and living systems. Examples include the water cycle, the carbon cycle, and the nitrogen cycle, which move water and elements through ecosystems. Biological cycles include the life cycle of organisms from birth to reproduction, the cell cycle that partitions cellular growth and division, and circadian rhythms that regulate daily physiological cycles. Seasonal and ecological cycles influence climate, habitats, and species interactions.

In mathematics and computer science, a cycle often denotes a closed path in a graph or a permutation consisting of a series of elements that return to the starting point. Cycles also appear in sequences and signals that repeat over fixed intervals, and in algorithms that detect or exploit periodicity.

Economically and socially, cycles describe recurrent patterns such as business cycles of expansion and contraction, fashion cycles, or other periodic phenomena in markets and cultures. The concept of a cycle emphasizes repetition, closure, and predictable pacing across contexts.