Thanks for the suggestions...
It is possible that it's just the boring F-G-H-thing mentioned above. But if anybody knows of another story, I'll be more than happy to hear it! :-)
During a basic lecture about system design the transfer function H(s) was discussed. The question was raised why the letter 'H' is used for this...
Sounds weird, but I did start to think about it... Indeed in many books and publications the letter H (besides sometimes G(ain)) is used to...