An instance variable in Java is a non-static variable that is specific to a certain object. It is declared within the curly braces of the class but outside of any method. Each instantiated object of the class has a separate copy or instance of that variable. The value of an instance variable can be changed by any method in the class, but it is not accessible from outside the class. Instance variables are usually initialized when the object is created, and they are variables that are specific to a certain instance of a class.

To declare an instance variable, you use the keyword, access specifier – “private,” “public,” etc., then the variable’s data type, followed by the variable’s name. For example, in the following code, "name" is an instance variable of the "Student" class:

public class Student {
    public String name;
}

Instance variables provide privacy and security for the data contained within them. They also make it easy to track state changes within an object, as each instance variable will keep track of its changes. Changes made in an instance variable using one object will not be reflected in other objects as each object has its own copy of the instance variable.

In summary, an instance variable is a variable that is specific to an object of a class in Java, and it is declared within the class but outside of any method. It is used to store data that is specific to an instance of a class, and it provides privacy and security for the data contained within it.