C++ encourages the use of meaningful names. Instead of vague identifiers like x or nav, it's better to use descriptive names such as cost_of_trip or costOfTrip
Only a-z, A-Z, 0-9 and _
Case sensitive
First character cannot be a digit
If first character is _, then undefined behaviour happens. Such names are reserved for the compiler.
C++ key words cannot be used
The length of the name of C++ variables is not limited. This is different from C, where the limit is 63.