周期 (zhōu qī) literally combines 周 (zhōu), meaning 'circle' or 'around', and 期 (qī), meaning 'period' or 'time span'. Together, they form a noun meaning 'cycle'—a recurring interval of time during which a process repeats itself. It’s widely used in scientific, technical, and everyday contexts to describe natural rhythms (e.g., menstrual cycle, life cycle), mechanical operations (e.g., engine cycle), or scheduled patterns (e.g., project timelines).

The word implies regularity and repetition over time, not just any duration. Unlike 时期 (shí qī, 'era' or 'period'), which refers to a one-off stretch of time, 周期 always suggests recurrence. It often appears with measure words like 一个 or modifiers like 长 (cháng, 'long') or 短 (duǎn, 'short') to indicate cycle length. You’ll frequently encounter it in biology, physics, economics, and even workplace planning.