Inertia
Definition
The tendency of objects to keep doing what they are already doing — staying still if still, or moving steadily if moving — unless a force acts on them.
Technical definition
The property of mass described by Newton's first law: in an inertial frame, an object maintains constant velocity unless acted on by a net external force. Quantitatively, an object's inertia is measured by its mass.
Example
When a bus brakes suddenly, your body lurches forward — not because a force pushed you, but because your inertia kept you moving while the bus slowed beneath you. The seatbelt supplies the force that finally changes your motion.
Inertia is not a force. Students often draw a forward 'inertia force' on free-body diagrams, but nothing is pushing the object forward — inertia is the absence of any need for a push. Real forces are things like friction, gravity, and the normal force. If it is not an interaction with another object, it does not belong on the diagram.