inertial frame
A reference frame in which Newton's laws hold in their simple form; one that is not itself accelerating.
Definition
An inertial frame is a reference frame in which a body with no forces on it moves in a straight line at constant speed. Equivalently, it is a frame in which Newton's laws of motion hold without modification. Any frame moving at constant velocity relative to an inertial frame is also inertial — the content of Galilean relativity.
Non-inertial frames — frames that are accelerating or rotating — break Newton's laws in their simple form. A passenger in a braking car sees coffee lurch forward; no force is actually pulling it. Describing the motion from inside the accelerating frame requires fictitious forces — centrifugal, Coriolis, and so on — that are bookkeeping for the frame's own acceleration.
The surface of the Earth is very nearly inertial for everyday mechanics but not exactly — it rotates once a day. Foucault's pendulum, hanging in the Panthéon in 1851, made the rotation visible. Einstein's special relativity kept the concept and upgraded Galilean relativity: the laws of physics, including the speed of light, are the same in every inertial frame.