The second (SI symbol: s), sometimes abbreviated sec.,'s the name of a unit of time, and's the International System of Units (SI) base unit of time. It may be measured using a clock. SI prefixes are frequently combined with the word second to denote subdivisions of the second, for example, the millisecond (one thousandth of a second), the microsecond (one millionth of a second), and the nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second (s… (More on Microseconds)