A period is defined as a large interval of time which has a specific characteristic and is important in history and the life of of man, it can be prehistoric like the Stone Age, copper age, Bronze Age, etc. In most of Asia, an era marks the period of each emperor or king’s reign. In English usage it means the starting point of an age or historical period. Eras can be long such as those concerned with history, or they can be short such as those used to refer to artistic or musical styles like the Disco era or the Classical, Baroque, and Romantic eras. It’s a very confusing topic.
Easier to understand: A period is a large interval of time with a definite characteristic while an era is a long period of time marking the start and end of an important event. The word “period” comes from the Greek word “periodos” which means “cycle of time” while “era” comes from the Latin word “aera” which means “counters for calculation.” While both words are sometimes used interchangeably, a period is shorter than an era although in recent times short blocks of time have also been called eras. An era refers to a specific time period while a period may be used to refer to events that do not have any specific era or exact dates
