Second

ssecmegasecondSI secondsecondsgigasecondmicrosecondsMsnanosecondsns
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.wikipedia
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SI base unit

base unitSI base unitsbase units
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.
The units and their physical quantities are the second for time, the metre for measurement of length, the kilogram for mass, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity.

Day

ddiurnaldays
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.
In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time.

Hour

htime of dayhr
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.
An hour (symbol: h; also abbreviated hr.) is a unit of time conventionally reckoned as 1⁄24 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.

Time

temporaldurationsequence of events
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.
The second (s) is the SI base unit.

International System of Units

SISI unitsSI unit
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (SI) is a much steadier timekeeper: it is defined by taking the fixed numerical value of the caesium frequency ∆ν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s −1.

Minute

minmmins.
The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.
The minute is a unit of time usually equal to 1⁄60 (the first sexagesimal fraction ) of an hour, or 60 seconds.

Leap second

leap secondsabolish the leap secondAnother second
Because the Earth's rotation varies and is also slowing ever so slightly, a leap second is periodically added to clock time to keep clocks in sync with Earth's rotation.
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (as measured by atomic clocks) and imprecise observed solar time (known as UT1 and which varies due to irregularities and long-term slowdown in the Earth's rotation).

Frequency

frequenciesperiodperiodic
The second is also part of several other units of measurement like meters per second for velocity, meters per second per second for acceleration, and per second for frequency.
One hertz means that an event repeats once per second.

Velocity

velocitiesvelocity vectorlinear velocity
The second is also part of several other units of measurement like meters per second for velocity, meters per second per second for acceleration, and per second for frequency.
Since the derivative of the position with respect to time gives the change in position (in metres) divided by the change in time (in seconds), velocity is measured in metres per second (m/s).

Clock

timepiecemechanical clockanalog clock
Analog clocks and watches often have sixty tick marks on their faces, representing seconds (and minutes), and a "second hand" to mark the passage of time in seconds.
It may also have a "second hand" which indicates the seconds in the current minute.

Isotopes of caesium

caesium-133 134 Cscaesium-134
Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (SI) is a much steadier timekeeper: it is defined by taking the fixed numerical value of the caesium frequency ∆ν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s −1.
The SI base unit the second is defined by a specific caesium-133 transition.

Atomic clock

atomic clocksatomiccaesium clock
Starting in the 1950s, atomic clocks became better timekeepers than earth's rotation, and they continue to set the standard today.
In 1967, this led the scientific community to redefine the second in terms of a specific atomic frequency.

Caesium

cesiumCsCs +
There have only ever been three definitions of the second: as a fraction of the day, as a fraction of an extrapolated year, and as the microwave frequency of a caesium atomic clock, and they have realized a sexagesimal division of the day from ancient astronomical calendars.
In 1967, acting on Einstein's proof that the speed of light is the most constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre.

Hertz

MHzkHzHz
Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (SI) is a much steadier timekeeper: it is defined by taking the fixed numerical value of the caesium frequency ∆ν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s −1.
The International Committee for Weights and Measures defined the second as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" and then adds: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, ν(hfs Cs) = 9 192 631 770 Hz."

Sexagesimal

sexagesimal systembase 60base-60
Sexagesimal divisions of the day from a calendar based on astronomical observation have existed since the third millennium BC, though they were not seconds as we know them today.
Al-Biruni first subdivided the hour sexagesimally into minutes, seconds, thirds and fourths in 1000 while discussing Jewish months.

Coordinated Universal Time

UTCUTC-3UTC-4
The international standard for timekeeping is Coordinated Universal Time (UTC).
Nearly all UTC days contain exactly 86,400 SI seconds with exactly 60 seconds in each minute.

Centimetre–gram–second system of units

CGScgs unitsCGS unit
BAAS formally proposed the CGS system in 1874, although this system was gradually replaced over the next 70 years by MKS units.
The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time.

System of measurement

Systems of measurementsystem of unitsHistorical weights and measures
In 1832, Gauss proposed using the second as the base unit of time in his millimeter-milligram-second system of units.
The current international standard metric system is the International System of Units (Système international d'unités or SI) It is an mks system based on the metre, kilogram and second as well as the kelvin, ampere, candela, and mole.

Hyperfine structure

hyperfinehyperfine splittinghyperfine interaction
Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (SI) is a much steadier timekeeper: it is defined by taking the fixed numerical value of the caesium frequency ∆ν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s −1.
One second is now defined to be exactly 9192631770 cycles of the hyperfine structure transition frequency of caesium-133 atoms.

Ground state

ground-stategroundelectronic ground
Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (SI) is a much steadier timekeeper: it is defined by taking the fixed numerical value of the caesium frequency ∆ν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s −1.

Ephemeris time

ephemeris secondephemeris time (ET)Ephemeris time – History
This resulted in adoption of an ephemeris time scale expressed in units of the sidereal year at that epoch by the IAU in 1952.
The ephemeris time of the 1952 standard leaves a continuing legacy, through its ephemeris second which became closely duplicated in the length of the current standard SI second (see below: Redefinition of the second).

MKS system of units

MKSMKS systemmetre–kilogram–second system
BAAS formally proposed the CGS system in 1874, although this system was gradually replaced over the next 70 years by MKS units.
The MKS system of units is a physical system of measurement that uses the metre, kilogram, and second (MKS) as base units.

List of non-standard dates

February 30January 00 January
The second was thus defined as "the fraction 1⁄31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time".
January 0 also occurs in the epoch for the ephemeris second, "1900 January 0 at 12 hours ephemeris time".

Seconds pendulum

a pendulum that has a period of 2 secondsone second pendulum
A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz.

Orders of magnitude (time)

kilosecondyoctosecondpetasecond
In some cases, the order of magnitude may be implied (usually 1), like a "second" or "year".