Time is a fundamental concept describing the progression of existence and events from the past through the present to the future.

Here are key aspects to understand it:

1. Measurement: We measure time using units like seconds, minutes, hours, days, and years. These are based on observable, cyclical phenomena such as the Earth's rotation and orbit.
2. Directionality: Time generally appears to move in one direction, often called the "arrow of time." We remember the past, experience the present, and anticipate the future.
3. Relativity (Physics): In modern physics, particularly Einstein's theory of relativity, time is not absolute. It is relative to the observer's motion and gravitational field. This means time can pass at different rates for different observers.
   • Special Relativity: Time slows down for objects moving at high speeds relative to an observer.
   • General Relativity: Time slows down in stronger gravitational fields.
4. Philosophical Perspective: Philosophically, time can be viewed as an inherent structure of the universe, a dimension in which events occur, or as a construct of the human mind used to order experience.
5. No Universal Agreement: Despite extensive study, there is no single, universally accepted definition of time that satisfies all scientific and philosophical inquiries.

In essence, time is the dimension that allows for change and sequence in the universe.