ANSWER
Since 1967 the second has been defined as the duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the fundamental state of the atom of cesium-133.
In slightly less scientific terms, all atoms "vibrate" at a very specific frequency. One second is defined as the time it takes a cesium atom to "vibrate" 9,192,631,770 times.
Prior to 1967, a second was defined as 1/86,400th or a "mean solar day".
Okay, since you asked... A "solar day" is what we normally think of as a day, which is to say, the time it takes the earth to rotate about its axis once, relative to the sun. (Not to be confused with a "sidereal day", which is the length of time it takes the earth to rotate about its axis once, relative to the rest of the galaxy, which is about four minutes shorter than a "solar day". Weird, but if you think about it long enough, it makes sense.) A "mean solar day" is just the average of all the "solar days" over the course of a year.
In case you were wondering, the second is metric. It was the third of seven standard base units of measure to be defined as part of the metric system, known today as the International System of Units (SI). The seven base units are meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), candella (luminous intensity), and mole (amount of substance).
WHO GOT IT RIGHT: Matthew Harrington, Marc Quinlivan, Marika Thiessen, James Forsyth, Bob Milligan, JP Weigt, and Andie J.
