A nanosecond (ns) is one billionth of a second (10⁻⁹ s). One nanosecond is to one second as one second is to 31.7 years.

The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns.

A nanosecond is equal to 1000 picoseconds or ¹⁄₁₀₀₀ microsecond. Because the next SI unit is 1000 times larger, times of 10⁻⁸ and 10⁻⁷ seconds are typically expressed as tens or hundreds of nanoseconds.

Times of this magnitude are commonly encountered in telecommunications, pulsed lasers and some areas of electronics. Some notable measurements in this range include:

Rear Admiral Grace Murray Hopper (December 9, 1906 – January 1, 1992) was an American computer scientist and United States Navy officer. A pioneer in the field, she was one of the first programmers of the Harvard Mark I computer, and developed the first compiler for a computer programming language. She conceptualized the idea of machine-independent programming languages, which led to the development of COBOL, one of the first modern programming languages. She is credited with popularizing the term "debugging" for fixing computer glitches (motivated by an actual moth removed from the computer). Due to the breadth of her accomplishments and her naval rank, she is sometimes referred to as "Amazing Grace." The U.S. Navy destroyer USS Hopper (DDG-70) was named for her, as was the Cray XE6 "Hopper" supercomputer at NERSC.

Hopper was born Grace Brewster Murray in New York City. She was the oldest in a family of three children. She was curious as a child, a life-long trait. At the age of seven she decided to determine how an alarm clock worked. She dismantled seven alarm clocks before her mother realized what she was doing; she was then limited to one clock. For her preparatory school education, she attended the Hartridge School in Plainfield, New Jersey. Rejected for early admission to Vassar College at age 16 (her test scores in Latin were too low), she was admitted the following year. She graduated Phi Beta Kappa from Vassar in 1928 with a bachelor's degree in mathematics and physics and earned her Master's degree at Yale University in 1930.