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inertial reference frame

(inertial frame of reference, Galilean frame of reference)
(frame of reference where no force means no acceleration)

An inertial reference frame (or inertial frame of reference or Galilean reference frame) is a frame of reference, e.g., set of coordinates, moving (or not) such that any object with no net forces on it is not accelerating, basically a frame of reference moving at constant speed, i.e., that is not itself accelerating. An example of something else (i.e., a non-inertial reference frame) is one that is rotating, e.g., a reference frame indicating one's position on Earth, which is rotating, thus, in which "standing still" on the surface actually includes a slight acceleration toward the central axis. One can sometimes work in such a non-inertial reference frame using the normal principles of dynamics by pretending the existence of forces that would explain the consequences of the frame's acceleration, examples of such fictitious forces being centrifugal force and the Coriolis force.

Further reading:

Referenced by pages:
frame of reference
gravity anomaly
ZAMO frame