An Olympiad is a period of four years, associated with the Olympic Games of the Ancient Greeks. During the Hellenistic period, beginning with Ephorus, Olympiads were used as a calendar epoch. By this reckoning, the first Olympiad lasted from the summer of 776 BC to that of 772 BC. By extrapolation, the 4th year of the 697th Olympiad begins in (Northern-Hemisphere) summer 2012.
Nowadays, a modern Olympiad refers to a period beginning January 1 of a year in which the Summer Olympics are due to occur, and lasting four years. The first modern Olympiad began during 1896, the second began during 1900, and so on (the 30th began during 2012: see the Olympic Charter).