The statement that a body remains in a state of rest or uniform motion unless acted upon by an external force is known as Newton's First Law, also referred to as the law of inertia. This fundamental principle of classical mechanics establishes that an object will not change its state of motion—whether at rest or moving with constant velocity—unless influenced by a net external force.
In practical terms, this law explains how objects behave in their environment. For example, if a ball is rolling on a frictionless surface, it will continue rolling indefinitely at the same speed and direction unless a force, like friction or a collision, acts upon it. This directly relates to our understanding of motion and how forces interact with objects, forming one of the foundational concepts in physics.
The other laws mentioned serve different purposes: Newton's Second Law relates force to mass and acceleration, Newton's Law of Gravitation describes the attractive force between masses, and Newton's Third Law states that for every action, there is an equal and opposite reaction. However, none of these explain the condition of a body remaining at rest or in uniform motion due to the absence of external forces, which is exclusively the focus of Newton's First Law.