equivalence principle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A principle central to the theory of general relativity stating that a gravitational field is locally indistinguishable from the effects of an inertial force, such as the acceleration provided by a rocket engine.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics any of several principles, in relativity, concerned with the uniformity of physical measurements in different frames of reference