Plain English Meaning
Things don't want to change what they're doing. If something is sitting still, it wants to keep sitting still. If it's moving, it wants to keep moving the same way. Heavier things resist change more than lighter things.
Full Definition
The tendency of an object to resist any change in its motion. An object at rest tends to stay at rest, and an object in motion tends to continue moving in a straight line at a constant speed unless acted on by an outside force. The greater an object's mass, the greater its inertia.
Context Anchor
Encountered in turn-radius discussions, especially when explaining why a faster airplane needs more room to turn.
Derivation
From the Latin 'iners,' meaning 'idle' or 'sluggish.' The idea is that matter is naturally lazy — it won't change what it's doing without being pushed. That's exactly the aviation meaning: the airplane resists changes in its motion, and the heavier it is, the more it resists.
Why Pilots Care
Inertia sets the amount of force ( and ) needed to change an airplane's , directly determining at a given .