Type Conditional compilation
Syntax {$UNDEF name}
Undefines a previously defined
conditional symbol. The symbol is forgotten for the remainder of the
compilation of the current source file or until it reappears in a $DEFINE
directive. The $UNDEF directive has no effect if name is already
undefined.
Conditional symbols defined with a command-line switch or through the Project|Options dialog are reinstated at the start of compilation of each unit source file. Conditional symbols defined in a unit source file are forgotten when the compiler starts on another unit.