Time is a familiar concept to everybody, yet very hard to define. Some scientists define time to be the progress of events that occur in an irreversible succession from the past, through the present, and into the future.

Time is one of the seven fundamental physical quantities in the International System of Units (SI), which can be used to define other physical quantities, such as speed. In everyday life, time is commonly measured in the units of second, minute, hour, day, week, month, and year, among others.