**Inertial mass**is a measure of the resistance of an entity to a change in its velocity relative to an inertial frame. Within classical physics the inertial mass of point particles is defined by means of the following equation for the subsequently described Machian thought experiment where particle 1 is taken as a unit (m

_{1}=1):

- m
_{i}a_{i1}= m_{1}a_{1i},

_{i}is the inertial mass of particle i, and a

_{i1}is the initial acceleration of particle i, in the direction from particle i to particle 1, in a volume occupied only by particles i and 1, where both particles are initially at rest one distance unit apart. There are no external forces, but the particles exert a force on each other.

The equation defines the inertial mass of particle i in terms of the assumed measurable mutually induced accelerations a_{i1} and a_{1i}. The remaining constraints on the accelerations, that the above defining equation still holds at different initial distances and when generated by the paring of particles with other than particle 1, can be taken as requirements for the experimental validity of the theory's dynamics, cf. momentum conservation.

In relativity theory, mass increases with velocity, and the rest mass is the inertial mass at v=0.