The concept of time is very complicated. In fact, not all people actually believe in the concept of time. Time is defined as the continued progress of events and all existence that happen in what is for now irreversible forward motion from the past to present to future. Time is made of measurements which are used to order events, quantify how fast things change, and to compare the time spent in or between events.
The concept of time is regularly referred to as the fourth dimension beyond the three other spatial dimensions. Time has been a crucial subject of study in science, religion and philosophy, but defining it in a way that works with all fields is very difficult.