What's inertia?
Inertia is the tendency of an object at rest to remain at rest; or the tendency of an object to continue moving if it is moving. In plain language, it's the tendency of an object to want to continue doing whatever it's doing.
I had a definition years ago given to me, and it still stands in the era of relativity:
Inertia is the property of matter that resists changes in its motion.