inertness, esp. with regard to effort, motion, action, and the like; inactivity; sluggishness.

Physics .

the property of matter by which it retains its state of rest or its velocity along a straight line so long as it is not acted upon by an external force.

an analogous property of a force

electric inertia.

Medicine/Medical . lack of activity, esp. as applied to a uterus during childbirth when its contractions have decreased or stopped.