In C++, a reference variable is an alias that refers to an existing variable. It provides an alternative name to an already existing variable in the program. Once a reference variable is declared, it can be used just like the original variable it refers to. Changes made to the reference variable affect the original variable, and vice versa.

Reference variables are declared using the ampersand (&) symbol. For example, to create a reference variable "ref" that refers to an integer variable "num", we can write:

int num = 5;
int &ref = num;

In this example, "ref" is a reference variable that refers to "num". Now we can use "ref" just like "num":

ref++;   // increments num to 6
cout << num << endl;   // outputs 6

Reference variables can be useful in many situations, such as passing variables to functions, returning variables from functions, or when we need to work with large data structures efficiently.