While mass is often called the coefficient of inertia, inertia *per
se* refers specifically to the force a body exerts upon any agency
which acts to change the body's motion. That phenomenon was referred
to as *vis inertiae* (the force of inertia) by Newton. Of course
Newton did not claim to have invented it, since the recognition of
the phenomenon antedates Newton (Descartes, Simon Stevin, Galileo).
Many other mechanics texts do refer to the first law as the law of
inertia or the principle of inertia (e.g. French, Fowles & Cassiday,
Davis), I think in recognition of the fact that it is not, strictly
speaking, Newton's, it shouldn't be referred to as his law.
It is interesting to note that when one writes F = ma and refers to
m as the coefficient of inertia, one is really recognizing that the
reaction force exerted by the body is its *vis inertiae*, and that
the equation ought to be written (as vectors) as F = - ma.