According to the perception of humans and many other races, time can be defined as the non-stopping, continuous and unidirectional process that enables change within reality. Since it is possible to assign temporal coordinates to distinct events, time is also defined as the fourth dimension, preceded by length, width and height. In contrast with the three spatial dimensions, motion through time, or aging, is inevitable and irreversible, occurring from past to future; with the present being the ever-changing temporal coordinate that one experiences.
In the present day, time is subject to study and theorization by physics. Even before the birth of physics however, the nature, properties, origin and the very existence of time has been discussed and questioned by philosophy and religion.
One of such topics is irreversibility, i.e. whether time is linear or cyclical. Ancient Greeks, Babylonians, Buddhists, Incas, Mayas and Hinduists regarded or regard that the former is true, that the universe undergoes endless cycles of destruction and rebirth. With a more scientific approach, stochastical recurrences of given physical states have been hypothesized to occur within extremelly long but finite periods of time, such as in Poincaré's theorem.
For everyday experience, one can assume that time is absolute, any given measure of time is equivalent everywhere. This was assumed by physicists such as Isaac Newton, but counteargued by others, such as mathematician Gottfried Wilhelm Leibniz.
Albert Einstein's theory of general relativity completely changed how humans understand time. According to his new view of classical mechanics, space and time are not absolute but relative to each observer, but they are nevertheless linked in what is called spacetime. Furthermore, the faster physical information displaces through space, the slower it does through time, and vice versa.
Essentially, the notion of absolute time can be said to be correct, but only when applied to bodies that have macroscopic masses and speeds well under that of light within a frame of reference. A body moving at significant fraction of the speed of light (relativistic speeds) will undergo Lorentz length contraction and time dilation; i.e. it will become shorter and age more slowly.
In fiction Edit
Conversely, there are numerous species that neither perceive nor move through time in a linear way necessarily, e.g., Time Lords, Prophets, and Vists, among others. Many advanced civilizations have mastered technology that allows them to travel through time freely or even manipulate and control time to some degree.