Short answers, pulled from the story.
The second is defined as exactly 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. This definition was adopted in 1967 and refers to a caesium atom at rest and unperturbed by external fields.
The word "second" comes from the Latin pars minuta secunda, meaning "second small part." The minute is the first small part (pars minuta prima) of the hour, dividing it into sixty. The second is the second such division, splitting the minute into sixty again.
Dutch scientist Christiaan Huygens invented the first pendulum clock in 1656, which was the first clock capable of accurately keeping time in seconds. Its pendulum was just under a meter long, giving it a one-second swing, and its escapement ticked every second.
The current generation of caesium atomic clocks is accurate to within one second in a few hundred million years. The most advanced caesium fountain clocks, such as NIST-F2 and PTB-CSF2, have a systematic uncertainty of fifty picoseconds per day.
Optical lattice clocks are the most accurate timekeepers currently in existence. A strontium optical lattice clock operating at 430 terahertz held the accuracy record during the 2010s, gaining or losing less than one second in fifteen billion years.
A day contains 86,400 seconds. A non-leap year contains 31,536,000 seconds. A Gregorian century averages 3,155,695,200 seconds, excluding any possible leap seconds. In astronomy, a Julian year is precisely 31,557,600 seconds.