Inertia is the property of matter that arises from the conservation of
energy in
Newton's laws of motion, which leads to the conservation of momentum, and thus, as stated by
Isaac Newton in one of his laws of physics, "An object in motion tends to stay in motion, and an object at rest tends to stay at rest unless acted upon by an external force."
More precisely, inertia is the property of an object that requires the application of an external
force in order to change the object's
velocity relative to an
inertial frame.
Inertial mass is a measure of inertial frame-independent inertia, and
momentum is a measure of inertial frame-relative inertia.
The underlying reason for this property of matter is unknown.