What is an instance variable ?

Attributes or Properties: Instance variables represent the attributes or properties of an object. For example, if you have a class representing a "Car," instance variables might include attributes like "color," "model," or "year."

Scope: The scope of an instance variable is limited to the instance of the class to which it belongs. It is not shared among instances of the class; each instance has its own set of values for its instance variables.

Initialization: Instance variables are often initialized in the constructor of the class. The constructor is a special method that gets executed when an object is created. It's responsible for setting up the initial state of the object, including the values of its instance variables.

Instance variables are declared/defined in a class but outside the constructor (s), method(s) and block(s). For memory management, when memory is allocated for an object in a heap, a slot for each of the instance variables is created. Further, see Instance vs Class Variable with respect to memory
Java Instance Variable: Use, Features, Instance vs Class[^]