Isochronism

Isochronism is a property of some oscillators, such as a pendulum. It means that the time taken to complete one oscillation is the same regardless of the amplitude, as long as the oscillation remains small.

It was famously observed by Galileo Galilei while he was in the Cathedral of Pisa in 1581.

Important detail

Isochronism is a conditional property. In reality, the amplitude does affect the period when the oscillations become large. Even so, it remains an important approximation and a useful property in physics.