period

The time it takes a repeating motion to return to the same state and start over. For a clock pendulum, one period is one full swing — left, right, back to where it started. Inverse to _frequency_ (`f = 1/T`). What makes a mechanical clock work is a period that does not change with amplitude or wind-down: tick exactly the same regardless of how widely the bob is swinging today. No real pendulum has that property exactly; small-angle pendulums _almost_ do.