Answer:
The IOC refers to an olympiad as a 4 year period beginning with the conclusion of the previous games and culminating with the Olympic games. Thus the first olympiad in Athens, Greece concluded in 1896 although there was no previous games. The 29th olympiad concluded with the games in Beijing, China in 2008. Incidently the 6th, 12th, and 13th retain their number even though there were no games due to World Wars 1 and 2. And the Winter Games do not follow this system at all being simply known as Olympic Winter Games numbered consecutively and NOT including those not held in 1940 and 1944.