Time

Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to compare the duration of events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience.

Time is often referred to as a fourth dimension, along with three spatial dimensions. Time has long been a major subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars. Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.

In physics, time is used to define other quantities, such as velocity and acceleration. The operational definition of time is that "time is what a clock reads". Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart.

Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms. Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human lifespans.

Philosophically, time is considered to be a fundamental structure of the universe—a dimension in which events occur in sequence. Spacetime is the model that combines space and time into a single interwoven continuum. The Big Bang theory suggests the beginning of the universe as we know it, including the start of time itself.