enumeration type::A user-defined data type with values that range over a set of named integral constants.
escape sequence::A specific combination of a backslash (\) followed by a letter or combination of digits, which represents white space and nonprinting characters within strings and character constants.
expression::A combination of operands and operators that yields a single value.
external variable::A variable that is defined outside any function in a C source file and is used in other source files in the same program.