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 physics, particularly in Newton's laws of motion, which describe how forces affect the motion of objects. Inertial frames are often associated with observers who are not accelerating. Inertial frames can be thought of as "non-accelerating" frames. For example, a person standing still on the ground or a spaceship traveling in deep space at a constant speed are both in inertial frames. In contrast, a car speeding up or turning is in a non-inertial frame because it is accelerating.