What is time, and how is it defined?

Time is the sequence in which events occur and the measure of intervals between them.

Scientific Definition

The SI system defines one second as exactly 9,192,631,770 oscillations of radiation from a caesium-133 atom the atomic clock definition.

Einstein showed Newton was wrong: time is not absolute it is relative.

• Time passes more slowly when you move faster (time dilation). An astronaut on the ISS ages very slightly slower than someone on Earth measurable and real.
• Time passes more slowly in stronger gravity. GPS satellite clocks tick faster than clocks on Earth's surface, and GPS systems must correct for this to stay accurate.

Einstein unified space and time into a single fabric called spacetime. Mass and energy bend this fabric what we experience as gravity is actually curvature in spacetime.

Why time flows in one direction the "arrow of time" remains one of the deepest unsolved questions in physics.