Frame or structural damage means the parts of the vehicle designed to carry load and absorb crash energy have been bent, crushed or cut. On most modern cars that structure is the unibody rather than a separate chassis.
Structural repair can be done properly, using measuring jigs to return the shell to factory tolerances. Done badly, it leaves a vehicle that tracks oddly, wears tyres unevenly and, more seriously, will not deform as designed in a future collision.
This is the category of damage that most justifies paying for an independent inspection, because the difference between a good and a bad repair is largely invisible without measurement.
Why it matters
It is the damage most likely to affect how the vehicle protects you in a crash, and it depresses resale value permanently.
How to check it
Have a body shop measure the structure. Look for weld marks that do not match factory patterns, crumpled or replaced rails, and fresh underbody sealant in one area only.
Does this apply to the car you are looking at?
Run the VIN or plate and see what is actually on record.
Related terms
Definitions are general guidance, not legal advice. Title rules, total loss thresholds and inspection requirements are set by each state and change over time, so confirm the position with the motor vehicle agency for the state the vehicle is titled in before relying on it.