The inertial mass of an object determines its acceleration in the presence of an applied force. According to Isaac Newton's second law of motion, if a body of mass m is subjected to a force F, its acceleration a is given by F/m
Inertial mass (which leads to momentum) is a measurement of a body's resistance to a change in velocity when an external force is applied to that body.


here is a video that further explain the term inertial
http://www.youtube.com/watch?v=by-7kkAu2Pg