In C++, a static function is a member function that is used to access only static data members. It cannot access non-static data members, nor can it call non-static member functions. It can be called even if no objects of the class exist, and it is used to maintain a single copy of the class member function across different objects of the class.

Static member functions are independent of any object of the class and can be called even if no objects of the class exist. They can also be accessed using the class name through the scope resolution operator. A static member function can access static data members and static member functions inside or outside of the class.

In plain C, a static function is a function that is only visible to other functions in the same file, more precisely the same translation unit. It is not related to C++ static member functions.

To summarize, a static function in C++ is a member function that can only access static data members and can be called even if no objects of the class exist. In contrast, a static function in plain C is a function that is only visible to other functions in the same file.