An era is a time of time marked by character, events, changes on earth, etc. When used in science, for example geology, an era denotes a clearly defined period of time of arbitrary but well-defined length, such as for example the Mesozoic Era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in social history, eras may for example denote a period of some monarch’s reign. In colloquial language, eras denote longer spans of time, before and after which the practices or fashions change to a significant degree. When era is extended to a calendar system, it is known as a calendar era. In Sanskrit or Indian culture eras are known as Yugas.