Actually, it is said that mass is a measure of an objects inertia, or resistance to acceleration. Momentum can be thought of as "inertia in motion." A stationary object has inertia and mass, but has zero momentum.
From wikipedia:
http://en.wikipedia.org/wiki/Inertia#Mass_and_.27inertia.27Mass and inertia
Physics and mathematics appear to be less inclined to use the original concept of inertia as "a tendency to maintain momentum" and instead favor the mathematically useful definition of inertia as the measure of a body's resistance to changes in momentum or simply a body's inertial mass.
This was clear in the beginning of the 20th century, when the theory of relativity was not yet created. Mass, m, denoted something like amount of substance or quantity of matter. And at the same time mass was the quantitative measure of inertia of a body.
The mass of a body determines the momentum P of the body at given velocity v; it is a proportionality factor in the formula:
P = mv
The factor m is referred to as inertial mass.
But mass as related to 'inertia' of a body can be defined also by the formula:
F = ma
By this formula, the greater its mass, the less a body accelerates under given force. Masses m defined by the formulae (1) and (2) are equal because the formula (2) is a consequence of the formula (1) if mass does not depend on time and speed. Thus, "mass is the quantitative or numerical measure of body’s inertia, that is of its resistance to being accelerated".
This meaning of a body's inertia therefore is altered from the original meaning as "a tendency to maintain momentum" to a description of the measure of how difficult it is to change the momentum of a body.
