Betterment is the term used when property is repaired which results in it being in a better condition than before the incident. Consequentially insurers may ask the client to contribute to the cost of the repair. The contribution is referred to as betterment.
Car insurance provides indemnity insurance. Indemnity insurance is the undertaking to restore the client to the same financial position they were in before the loss occurred. If, before the loss, the car had 2 thread bare tyres and now, as a result of the repairs, it has 2 brand new tyres the client is asked to contribute to the cost of putting the car into a better position. (If the tyres had contributed to the cause of the incident the claim could be repudiated on the grounds that the car was not in a roadworthy condition.)