tsükkel

Tsükkel is the Estonian term for a recurring sequence or period after which events repeat or return to a similar state. The word derives from the Greek kyklos meaning “circle” and entered Estonian via Latin cyclus, gaining wide use in science and everyday language to denote any pattern of repetition with a definable duration.

In practice, tsükkel may describe natural, biological, social or technical processes. A cycle has a beginning and an end, may include phases, and can be observed or measured by its period—the time between repeats. Some cycles are strict and regular, while others are approximate or irregular.

Natural cycles include circadian rhythms, the day–night cycle, seasonal cycles and lunar cycles. Biological cycles encompass the cell cycle and various metabolic cycles. Economic cycles describe fluctuations in activity such as expansions and contractions in a market. In engineering and physical sciences, thermodynamic cycles, electrical cycles and similar cyclic processes are analyzed to understand efficiency and performance.

Tsüklid can be periodic, repeating at regular intervals, or quasi-periodic, with some variability in their timing. They may be driven by internal regulation, external forcing, or a combination of both. The concept is widely used in modeling, prediction, analysis and control across disciplines, and it also appears in everyday language to refer to routines or repeating events, such as training cycles or maintenance schedules.