Time interval is a term used to describe a length of time between two points. It can be measured in terms of DISTANCE, ERA, PERIOD, or DURATION. DISTANCE is a measure of the space between two points, usually expressed in terms of miles or kilometers. ERA is a PERIOD of time marked by particular characteristics or events, such as the Stone Age or the Industrial Revolution. PERIOD is a length of time that something lasts, such as a school year or a presidential term. DURATION is the amount of time something continues, such as a movie or a song.