A second is the base unit of time in the International System of Units (SI). It can be subdivided into smaller units, such as milliseconds, microseconds, and nanoseconds. A millisecond is one-thousandth of a second, a microsecond is one-millionth, and a nanosecond is one-billionth. These subdivisions are essential in fields like computing and telecommunications, where precise timing is crucial. In addition to these common subdivisions, there are even smaller units like picoseconds and femtoseconds. A picosecond is one-trillionth of a second, while a femtosecond is one-quadrillionth. These tiny measurements are often used in physics and chemistry to study rapid processes, such as molecular reactions.