Zeitgeist: Zeitgeist means spirit of the time. It is the dominant perspective or ideology of a time. In the early days of psychology the "zeitgeist" was Structuralism, and then Functionalism, and so on. Whatever the primary perspective of the day is/was, is considered the zeitgeist.

Zygote: When a female (human) egg is fertilized, it is no longer classified simply as an egg, but is now considered a zygote. For two weeks, the zygote goes through an amazing amount of development that occurs in the form of cell division. The cells of the zygote divide very rapidly, thus doubling during each division (we all start as just a single celled organism, but this changes very quickly).