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.

जडत्व म्हणजे एखादी वस्तू त्याच्या वर्तमान गतीमध्ये चालू राहणे जोपर्यंत काही बलामुळे तिचा वेग किंवा दिशा बदलत नाही. न्यूटनने त्याच्या गतीच्या पहिल्या नियमात वर्णन केल्याप्रमाणे हा शब्द "जडत्वाच्या तत्त्वासाठी" न्यूटनने आपल्या पहिल्या नियमात सांगितला आहे.