Frame of reference: a coordinate system relative to which motion is described or observed
Inertial frame of reference: a frame of reference that moves at a zero or constant velocity; a frame in which the law of inertia holds
Non-inertial frame of reference: a frame of reference that accelerates with respect to an inertial frame; the law of inertia does not hold
Apparent weight: the magnitude of the normal force acting on an object in an accelerated (non-inertial frame of reference