Equivalence principle: The fundamental postulate of Sir Einstein’s general theory of relativity that posits that acceleration is basically indistinguishable from the gravitational field. In another words, when you are in an elevator that is utterly sealed and protected from the outside, and hence you can’t "peek outside," then when you feel a force (that is weight), it is basically not possible for you to say whether the elevator is present in a gravitational field, or whether the elevator has rockets joined to it and is accelerating "upward."
However that in practical conditions -- say, sitting in a closed room -- it would be probable to determine whether the acceleration felt was due to the uniform thrust or due to the gravitation (state, by computing the gradient of the field; if non-zero, it would point out a gravitational field instead of thrust); though, these differences could be made randomly small. The idea at the back is the equivalence principle is that it acts about the vicinity of a point, instead of over macroscopic distances. This would be not possible to state whether or not a given (random) acceleration field was caused by the thrust or gravitation by the use of physics by only.
The equivalence principle forecasts interesting general relativistic consequences since not only are the two indistinguishable to human observers, however also to the Universe as well -- any effect which occurs whenever an observer is accelerating must also occur in a gravitational field, and vice-versa.