The second is the base unit of time in the International System of Units. Its symbol is s. It is defined by fixing the frequency of the caesium-133 hyperfine transition at exactly 9,192,631,770 cycles per second, so a second is the duration of that many oscillations.
The name records a division that no longer exists in practice. Medieval astronomers divided the hour into sixty minutae primae, first small parts, and each of those into sixty minutae secundae, second small parts. English kept the first as minute and the second as second, leaving the ordinal in the name of a unit almost nobody thinks of as ordinal.
For most of history the second was defined from the rotation of the Earth, as one 86,400th of a mean solar day. That definition failed once clocks became better than the planet: the Earth's rotation is irregular, slowing over centuries from tidal friction and wobbling on shorter timescales. The atomic definition adopted in 1967 severed the link, which is why leap seconds are occasionally inserted to keep civil time aligned with the sun.
Every other time unit is now built on it, and the second underpins most of the rest of the SI as well. The metre is defined through the speed of light in metres per second, the kilogram through the Planck constant which carries a second in its units, and the ampere through the elementary charge per second. Caesium fountain clocks realise the unit to a few parts in ten to the sixteenth, and optical clocks now do better still.
Because atomic clocks keep better time than the planet does, the two have to be reconciled. The Earth's rotation is slowing irregularly, so a day is now a millisecond or two longer than 86,400 atomic seconds, and the difference accumulates. Since 1972 the answer has been the leap second: an extra second inserted at the end of a June or December, announced months in advance, which has happened twenty-seven times. Computer systems handle the repeated timestamp badly enough that outages have been traced to it, and in 2022 the world's metrology bodies voted to abandon the practice by 2035, letting civil time drift from solar time instead.
One second equals 1000 milliseconds, one sixtieth of a minute, or about 0.000277778 hours.
| Unit | Symbol | 1 Second equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Microsecond | µs | 1000000 µs | 0.000001 s | Seconds to Microseconds |
| Millisecond | ms | 1000 ms | 0.001 s | Seconds to Milliseconds |
| Centisecond | cs | 100 cs | 0.01 s | Seconds to Centiseconds |
| Decisecond | ds | 10 ds | 0.1 s | Seconds to Deciseconds |
| Minute | min | 0.0166666666667 min | 60 s | Seconds to Minutes |
| Hour | hr | 0.000277777777778 hr | 3600 s | Seconds to Hours |
| Day | d | 0.0000115740740741 d | 86400 s | Seconds to Days |