Equivalence principle
Gravity and acceleration are locally indistinguishable — Einstein's key to general relativity.
The equivalence principle observes that a person in free fall feels no gravity, and that being accelerated feels exactly like standing in a gravitational field. Locally, you cannot tell the two apart.
Einstein later called this realisation of 1907 “the happiest thought of my life”; it became the foundation of general relativity.
Source: Wikipedia: Equivalence principle — Read the full article on Wikipedia.