Time dilation was first introduced by Albert Einstein's the theory
of relativity.Before Einstein everybody thought time was constant. In his theory of relativity, time dilation is an actual difference of elapsed time between two events as measured by observers either moving relative to each other(special relativity) or differently situated from gravitational masses(general relativity).
Special Relativity
When two observers are in relative uniform motion and uninfluenced by any gravitational mass, the point of view of each will be that the other's (moving) clock is ticking at a slower rate than the local clock. The faster the relative velocity, the greater the magnitude of time dilation.This is velocity based time dilation,so greater the velocity greater the time dilation.
General Relativity
Gravitational time dilation is a time dilation, an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass. The stronger the gravitational potential (the closer the clock is to the source of gravitation), the slower time passes and viceversa.This is the gravitational potential based time dilation,so greater the gravitating mass greater the time dilation.
