An inertial frame is a reference frame in which an object either remains at rest or moves at a constant velocity unless acted upon by an external force. This concept is fundamental in classical mechanics, as it simplifies the analysis of motion. Inertial frames are often associated with Newton's first law of motion, which states that an object will not change its state of motion unless a force is applied. Inertial frames can be thought of as non-accelerating frames. For example, a train moving at a constant speed on a straight track is an inertial frame, while a car accelerating or turning is not. The laws of physics, such as those formulated by Isaac Newton, hold true in all inertial frames, making them essential for understanding motion in the universe.