What kind of reference frame is one in which Newton's first law holds?

The kind of reference frame in which Newton’s first law holds is an inertial frame. In such a frame, an object with zero net force remains at rest or moves with constant velocity in a straight line.

“Inertial” does not mean that the frame itself is motionless. Two frames moving at constant velocity in straight lines relative to one another can both be inertial. A passenger in a smoothly moving train and an observer on the ground can each use Newton’s laws without introducing fictitious forces, provided the train is not accelerating or turning.

Accelerating and rotating frames are non-inertial. In those frames, apparent forces such as the centrifugal and Coriolis forces must be included to describe motion correctly. A rotating amusement-park ride is an everyday example: objects appear to veer or be pushed outward because the reference frame is rotating.

Newton originally imagined an absolute space, while modern mechanics treats inertial frames as a family of equivalent frames. Special relativity later refined these ideas, but inertial frames remain central to classical mechanics.