Inertia is defined as «a property of matter by which it remains... in uniform motion in the same straight line unless acted upon by some external force.» (curolegal.com)
By contrast Newton had inert mass remaining perpetually at rest or in uniform motion until another body altered it from without through efficient causality; the earth revolves about the sun in one solar system in the midst of an infinite universe. (faith.org.uk)
The first law states, «Every object continues in its state or rest, or of uniform motion in a straight line, unless compelled to change that state by external forces acted upon it.» (datingadvice.com)