Zeitgeist is the spirit or essence of a particular time. In the 1920s, flappers and speakeasies contributed to that era's Zeitgeist.
Zeitgeist is a word that comes straight from German — zeit means "time" and geist means spirit, and the "spirit of the time" is what's going on culturally, religiously, or intellectually during a certain period. Think about how something like Woodstock symbolized the 1960s: Woodstock was part of the Zeitgeist of the 1960s. Whatever seems particular to or symbolic of a certain time is likely part of its Zeitgeist.