In C++, a variable definition is a declaration that also allocates memory for the variable. The syntax for defining a variable is similar to that for declaring a variable, but with an optional initialization value:

data_type variable_name = initial_value;

Here, "data_type" is the type of data that the variable will store, "variable_name" is the name you choose for the variable, and "initial_value" is an optional initial value for the variable.

For example, to define and initialize an integer variable named "myNumber" to the value 42, you would write:

int myNumber = 42;

This statement not only declares the variable "myNumber" of type "int", but also reserves memory for it and initializes it with the value 42.

If you declare a variable without defining it, you can use its name in the program, but it will not have any memory allocated for it. This can cause errors or undefined behavior if you try to use the variable's value.