m25 is a macro processor, similar to M4. (The M4 and the M25 are both British motorways; I grew up near the M25.)
Compared to M4, m25 has simpler expansion rules, and a more straightforward TeX-like syntax. It's written in Python: you can use it from the command line or as a module in other Python programs, and you can call out from a macro to Python.
I originally wrote m25 because I wanted something better than M4 for building web pages with; the page you're reading now is generated using m25. Other uses I've found for it include machine-specific expansion of configuration files, and generating email feedback when marking student work.