Inertia is the tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes its speed or direction to change. It is one of the fundamental principles in classical physics, and described by Isaac Newton in his first law of motion. It is one of the primary manifestations of mass, one of the core quantitative properties of physical systems. Newton writes:

LAW I. Every object perseveres in its state of rest, or of uniform motion in a right line, except insofar as it is compelled to change that state by forces impressed thereon.

ఏ బాహ్య బలం పనిచేయకపోతే నిశ్చల స్థితిలో ఉన్న వస్తువు అదే స్థితిలోనూ సమవేగంతో ఋజు మార్గంలో గమించే వస్తువు అదే గమన స్థితిలోనూ ఉండిపోయే వస్తువు ధర్మాన్ని జడత్వం అంటారు.