Static data members are class members that are declared using static keywords. A static member has certain special characteristics. These are:

• Only one copy of that member is created for the entire class and is shared by all the objects of that class, no matter how many objects are created.
• It is initialized before any object of this class is being created, even before main starts.
• It is visible only within the class, but its lifetime is the entire program

In C++, a static data member is a variable that belongs to a class rather than an instance of the class. It can be thought of as a global variable that is associated with the class, rather than the objects created from that class.

To declare a static data member in C++, the static keyword is used before the data type in the class definition. For example: 

class Example {
  public:
    static int staticMember;
};

int Example::staticMember = 0; // initialization outside the class definition

In this example, the static data member staticMember is declared within the class Example. The value of this member is shared among all instances of the class, and can be accessed without the need to create an object of the class.

Static data members are useful in situations where you need to maintain a common value across all instances of a class. They can also be used to count the number of objects created from a class, for example.

It's important to note that static data members can only be accessed within the class definition, and not through instances of the class. They are initialized outside the class definition and can be accessed using the scope resolution operator (::) with the class name.