inertia (Ĭnûr´shə), in physics, the resistance of a body to any alteration in its state of motion, i.e., the resistance of a body at rest to being set in motion or of a body in motion to any change of speed or change in direction of motion. Inertia is a property common to all matter. This property was first observed by Galileo and restated by Newton as his first law of motion, sometimes called the law of inertia. Newton's second law of motion states that the external force required to affect the motion of a body is proportional to that acceleration. The constant of proportionality is known as the mass, which is the numerical value of the inertia; the greater the inertia of a body, the less is its acceleration for a given applied force.

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inertia Property possessed by all matter that is a measure of the way an object resists changes to its state of motion. Isaac Newton formulated the first law of motion, sometimes called the law of inertia, stating that a body will remain at rest or in uniform motion unless acted upon by external forces.

inertia (in-er-shă) n. (in physiology) sluggishness or absence of activity in certain smooth muscles. uterine i. inertia of the muscular wall of the uterus during labour, making the process excessively long. It may be present from the start of labour or it may develop because of exhaustion following strong contractions.