Remarks 

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.