Why is Newton's first law called inertia?

Newton's first law is the same law rephrased as follows: "Everybody continues to be in its state of rest or of uniform motion along a straight line unless compelled by some other body to act otherwise." In other words, it states that "If force on a body is zero, its acceleration is zero." Aristotle's fallacy was believed for years until Galileo finally extrapolated simple ideas of motion of bodies falling on inclined planes and arrived at the law of inertia, which destroyed Aristotlean mechanics.

This law elucidates the concept of inertia: it is the property of a body that resists any change in its state of rest or of uniform motion.

Newton's first law is called the law of inertia because it describes how an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity, unless acted upon by an external force. Inertia is the tendency of an object to resist changes in its motion.