SI prefixes are frequently combined with the word second to denote subdivisions of the second , e.g. , the millisecond ( one thousandth of a second ) and nanosecond ( one billionth of a second ) . SI prefixes are frequently combined with the word second to denote subdivisions of the second , e.g. , the millisecond ( one thousandth of a second ) , the microsecond ( one millionth of a second ) , and the nanosecond ( one billionth of a second ) .