Question

Name the S.I unit of time and define it.

Answer 1

Hint Time is the everlasting continued progress of existence and events that take place in a seemingly irreversible journey from the past, through the present, and into the future. A unit of time is the time interval, used as a standard way of measuring the time lapsed.

Complete Step-by-step solution
The S.I. unit of time is second which is represented as $s$.
The second is defined as the time taken by the Cesium-$133$ atom to complete $9192631770$ oscillations. Where one oscillation means a single transition from the ground state.

Additional information
Earlier units of time were defined by the motion of astronomical objects for example,
Sun based: In this, the year was defined as the time for the earth to revolve around the sun. Year-based units include the Olympiad $\left( {4years} \right)$, the lustrum $\left( {5years} \right)$, the induction $(15years)$, the decade $\left( {10years} \right)$, the century $(100years)$ and the millennium $(1000year)$.
Moon-based: In this, the month was defined on the moon's orbital period around the earth.
Earth-based: In this, the time is based on how long the earth took to rotate about its axis, as observed on a sundial. Units made from this base include the week at seven days and the fortnight at $14$ days. Subdivisions of the day in this format include the hour ($\dfrac{1}{{24}}$ of a day), this was further divided into minutes and finally into seconds. Later on, the second became the international standard unit (SI units) for science.
There are still more units to define time apart from the units discussed above.

Note
There are various ways to measure time but the worldwide accepted method is the way to measure the time elapsed during $9192631770$oscillations of Cesium-$133$atom. The year is defined as $365.2425\;$days due to which it has to be adjusted with leap days and leap seconds which decreases its reliability towards defining the S.I. unit.