Inertia is a property of an object that resists changes in its motion, state or rest. In other words, it refers to the tendency of an object to maintain its current state unless acted upon by external forces. This can be observed when an object at rest tends to remain at rest and an object in motion tends to continue moving with a constant velocity until some force causes it to change direction or speed.
Inertia is also used as a measure of how difficult it is to start, stop or change the motion of an object. The more mass an object has, the greater its inertia will be and the harder it will be to move. This concept is often referred to as "mass" or "inertial mass".
In physics, inertia plays a significant role in understanding the behavior of objects undergoing motion. It forms the basis for many fundamental principles such as Newton's First Law of Motion which states that an object will remain at rest or in uniform linear motion unless acted upon by external forces.