A data member, also known as an instance variable, is a component of an object. Objects are miniature programs that consist of both code and data. It is these data items that are referred to as data members.

A data member is not accessible directly via the object. Rather, access is through the use of what is termed a member function. A member function is the code component of the object.

The qualities of an object can be divided into two categories—form, and behavior. The form of a class of objects comprises the qualities that separate objects in a certain class from objects in another class. In the Java programming language, the data members specify the form of an object. In other words, in a given class, the data members of the constituent objects represent those qualities that are common to the objects in that class.

Each data member has an identifier. The identifier is the name of the data member, expressed as a unique combination of letters, digits, and underscore characters. An identifier always begins with a letter.