what is pure virtual function in C#?

When you define only function prototype in a base class without and do the complete implementation in derived class. This base class is called abstract class and client won’t able to instantiate an object using this base class.
A pure virtual function is a function that must be overridden in a derived class and need not be defined.
A virtual function is declared to be "pure" using the curious "=0"
syntax:
class Base
{
public: void f1(); // not virtual
virtual void f2(); // virtual, not pure
virtual void f3() = 0; // pure virtual
};