[oop] What is the difference between an Instance and an Object?

Objects and instances are mostly same; but there is a very small difference. If Car is a class, 3 Cars are 3 different objects. All of these objects are instances. So these 3 cars are objects from instances of the Car class.

But the word "instance" can mean "structure instance" also. But object is only for classes.

All of the objects are instances. Not all of the instances must be objects. Instances may be "structure instances" or "objects". I hope this makes the difference clear to you.