The term 'distinguishable period' can be represented by the 3-letter word 'ERA'. An ERA is a period of time in history that is marked by certain characteristics and events. It is usually used to refer to a long period of time, such as the Stone Age or the Industrial Revolution. An ERA can also refer to a shorter period of time, such as the Roaring Twenties or the Digital Age. In either case, an ERA is a distinct and recognizable period of time that can be distinguished from other periods.