noun (n)

• a disposition to remain inactive or inert(noun.attribute)

  Example:
  He had to overcome his inertia and get back to work.
  

  Synonym:
  inactiveness, inactivity
  Antonym:
  activeness, activity
  

  source: wordnet30
• (physics) the tendency of a body to maintain its state of rest or uniform motion unless acted upon by an external force(noun.phenomenon)

  source: wordnet30
• That property of matter by which it tends when at rest to remain so, and when in motion to continue in motion, and in the same straight line or direction, unless acted on by some external force; -- sometimes called vis inertiæ.(noun)

  source: webster1913