From AnthroWiki

A Manvantara (Sanskritमन्वन्तर, contraction of manu-antara, literally the "life or age of a manu") or world day is the external revelation of a planetary state of evolution. In the course of its development, each world system undergoes seven such externally revealed states, each of which is separated from the others by a world night, a so-called Pralaya, during which the world system completely withdraws from external revelation. Each Manvantara is guided by a specific manu.