In C++, a variable is a name given to a memory location that stores data values. It is the basic unit of storage in a program, and the value stored in a variable can be changed during program execution. A variable is only a name given to a memory location, and all the operations done on the variable affect that memory location. In C++, all variables must be declared before use.

To declare a variable in C++, you need to specify its type and name. There are different types of variables in C++, such as int, double, char, string, and bool. The name of the variable should not contain any whitespace or special characters, and it must begin with a letter of the alphabet or an underscore. You cannot use C++ keywords as variable names.

A variable declaration refers to the part where a variable is first declared or introduced before its first use. A variable definition is a part where the variable is assigned a memory location and a value. Most of the time, variable declaration and definition are done together.

In summary, a variable in C++ is a named memory location that stores data values. It must be declared before use, and its name should follow certain rules.