Plain English Meaning

Things don't start moving, stop moving, speed up, slow down, or change direction on their own. Something has to push or pull them to make that happen. And the heavier the thing, the harder it is to change what it's doing.

Full Definition

The principle that a body at rest will remain at rest, and a body in motion will continue moving in a straight line at a constant speed, unless acted upon by an outside force. The tendency of an object to resist any change in its state of motion is called inertia.

Context Anchor

Seen in instrument flying when discussing how an aircraft responds to forces, control inputs, turns, acceleration, and recovery from unusual attitudes.

Derivation

Inertia comes from the Latin iners, meaning 'idle' or 'sluggish' -- the same as 'inert.' It captures the idea that , alone, just keeps doing whatever it was already doing. formalized this as the first of his three laws of motion in 1687.