Emily Dickinson Lexicon

VIS-IN-ER'TIAE, n. [VIS IN-ER'TIÆ; L.]

1. The resistance of matter to change as respects motion. There are four conditions under which matter resists change as respects motion; as, 1) when it is brought from rest to motion; 2) when it is brought from motion to rest; 3) when the direction of the motion is changed; and 4) when the velocity is changed. Vis inertiæ and inertia are not strictly synonymous. The former implies the resistance itself, which is given, while the latter implies merely the property, by which it is given. Gravitation is always exactly proportioned to inertia.
2. Inertness; inactivity.