Newton's first law of motion describes:

1. Newton's first law says a body keeps its state of rest or uniform motion in a straight line until an external force acts on it. 2. This tendency to resist any change in state is called inertia. 3. Action and reaction pairs are described by Newton's third law. 4. So Newton's first law introduces the property of inertia. _Source: Selina Concise Physics ICSE Class 9, Ch 3 'Laws of Motion' (aplustopper.com extract)_