How long is a second officially defined?
Quick Answer: A second is officially defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.
This atomic definition of the second was adopted in 1967 by the International System of Units (SI). It replaced older definitions that relied on Earth's rotation, which is not perfectly stable.
Using cesium atoms allows for an extremely precise and stable measurement of time. Atomic clocks based on this standard are accurate to billionths of a second per day, helping keep global time consistent.