Virtual method table

A virtual method table (VMT), virtual function table, virtual call table, dispatch table, vtable, or vftable is a mechanism used in a programming language to support dynamic dispatch (or run-time method binding).

Whenever a class defines a virtual function (or method), most compilers add a hidden member variable to the class which points to an array of pointers to (virtual) functions called the virtual method table (VMT or Vtable). These pointers are used at runtime to invoke the appropriate function implementations, because at compile time it may not yet be known if the base function is to be called or a derived one implemented by a class that inherits from the base class.

Suppose a program contains several classes in an inheritance hierarchy: a superclass, Cat, and two subclasses, HouseCat and Lion. Class Cat defines a virtual function named speak, so its subclasses may provide an appropriate implementation (e.g. either meow or roar).

When the program calls the speak function on a Cat reference (which can refer to an instance of Cat, or an instance of HouseCat or Lion), the code must be able to determine which implementation of the function the call should be dispatched to. This depends on the actual class of the object, not its declared class Cat. The class can not generally be determined statically (that is, at compile time), so neither can the compiler decide which function to call at that time. The call must be dispatched to the right function dynamically (that is, at run time) instead.

...
Wikipedia