(Sanskrit महमन्वन्तर) “The Great Day.” A manvatara is a period of activity, thus adding maha- "great" to the beginning means an even greater or longer period of activity, as opposed to a Mahapralaya, a cosmic night or period of rest. Manvataras and mahamanvantaras are relative, not fixed periods or times. Everything has periods of activity and periods of rest, and those times are relative: our own periods of activity and sleep vary widely; moreover, we also have "great" times of activity (such as when engaged in a project) and "great" times of repose (like retirement, long vacations, illnesses, etc). The same is true of atoms, cells, planets, solar systems, universes....

"Truthfully, the quantities of years assigned to a cosmic day are symbolic. The cosmic night arrives when the ingathering of the perfect souls is complete, which means, when the cosmic day is absolutely perfected." —Samael Aun Weor, The Gnostic Bible: The Pistis Sophia Unveiled

"I was absorbed within the Absolute at the end of that Lunar mahamanvantara, which endured 311,040,000,000,000 years, or, in other words, an age of Brahma." —Samael Aun Weor, The Revolution of Beelzebub