The tendency of an object to resist a change in its motion is inertia.
Inertia means that an object at rest tends to remain at rest, while an object in motion tends to keep moving in a straight line at constant speed unless an external force changes its velocity. Mass measures how much inertia an object has: pushing a loaded truck requires more effort than pushing an empty cart.
The idea became central to classical physics through Galileo Galilei’s studies of motion and was formalized by Isaac Newton in the first law of motion, published in 1687. Everyday friction and air resistance often hide inertia by slowing moving objects, which can make it seem as though continuous force is needed to keep something moving.
Inertia is not the same as momentum. Momentum depends on both mass and velocity, while inertia is the resistance to changes in motion. Rotational inertia similarly describes resistance to changes in spinning motion, and depends on how mass is distributed around an axis.