1. The tendency of a body to resist acceleration: "In physics, vis inertiae is the resistance of a force of inertia; by extension, this may be applied to resistance in matters of social progress or change. It explains why a plan set in motion is difficult to stop, and vice versa.
2. The resistance of matter: "Vis inertiae exists when a body at rest is set in motion, or a body in motion is brought to rest, or when it has its motion changed, either in direction or in velocity."