In C++, a copy constructor is a special member function used to create a new object by copying the values of an existing object of the same class. It ensures that the new object is a duplicate with its own memory. The syntax for a copy constructor is typically `ClassName(const ClassName &other)`.

The copy constructor has the following characteristics:

1. It has the same name as the class it belongs to.
2. It takes one argument of the same class type (a reference to an object of the same class) as its parameter.
3. It is usually declared as a public member function.
4. It is automatically called in certain situations, such as when you pass an object by value, return an object by value from a function, or when initializing one object with another of the same class.