Instance variable
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An instance variable is a variable which available only to one instance of a class. It is the object's own property. For example, a particular square may be 4 inches wide. Other squares may have different widths.
A class variable, by contrast, is available to all instances of a class. It is a property which is the same for all objects. For example, the interior angle of every square is 90 degrees, no matter how wide the square is.