Inertia

INERTIA, in physics, is a property of matter.

Isaac Newton has defined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica, which states: The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line.

Simply speaking, inertia is that property of bodies by which they remain in a state of rest or of motion in a straight line till disturbed by a force moving them in the one case or arresting them in the other.