Question

What is the name for the inherent property of matter to retain its state of rest or uniform motion in a straight line unless acted upon by external forces?

Answer 1

This property of the body to retain its state is known as Inertia.

The law of inertia states that it is the tendency of an object to resist a change in motion. According to Newton's first law of motion, an object will stay at rest or stay in motion (i.e. 'maintain its velocity' in modern terms) unless acted on by a net external force, whether it results from gravity, friction, contact, or some other source.

Now lets see this mathematically
F = ma

where m remains constant so external force is directly proportional to a means no force no acceleration/retardation means velocity remains same.

Comments

According to Newton's third law of motion, an object will stay at rest or stay in motion (i.e. 'maintain its velocity' in modern terms) unless acted on by a net external force, whether it results from gravity, friction, contact, or some other source.
Sir this is the first law of motion.