inertial mass

WordReference Random House Unabridged Dictionary of American English © 2025

iner′tial mass′, [Physics.]

1. Physics the mass of a body as determined by the second law of motion from the acceleration of the body when it is subjected to a force that is not due to gravity.
2. Physics the measure of the property of inertia. Cf. Eötvös experiment, inertia (def. 2a), gravitational mass. 

Collins Concise English Dictionary © HarperCollins Publishers::

inertial mass noun

1. the mass of a body as determined by its momentum, as opposed to gravitational mass. The acceleration of a falling body is inversely proportional to its inertial mass but directly proportional to its gravitational mass: as all falling bodies have the same constant acceleration the two types of mass must be equal

See gravitational mass