Brahma's Night. A period of equal duration, in which Brahma is said to be asleep. Upon awakening he recommences the process, and this goes on for an AGE of Brahma composed of alternate "Days" and "Nights," and lasting for 100 years of 2,160,000,000 each. It requires fifteen figures to express the duration of such an age, after the expiration of which the Mahapralaya or Great Dissolution sets in, and lasts in its turn for the same space of fifteen figures.
Source: H.P.Blavatsky - The Key to Theosophy, The theosophical glossary