What is a modifier?

What is a modifier?
A modifier, also called a modifying function is a member function that
changes the value of at least one data member. In other words, an
operation that modifies the state of an object. Modifiers are also
known as ‘mutators’. Example: The function mod is a modifier in the
following code snippet:

class test
{
   int x,y;
   public:
    test()
    {
        x=0; y=0;
    }
 void mod()
    {
        x=10;
        y=15;
    }
};