The M4 package contains the M4 processor
M4 is a macro processor. It copies input to output expanding macros as it goes. Macros are either builtin or user-defined and can take any number of arguments. Besides just doing macro expansion m4 has builtin functions for including named files, running UNIX commands, doing integer arithmetic, manipulating text in various ways, recursion, etc. M4 can be used either as a front-end to a compiler or as a macro processor in its own right.