As I'm a wee bit obsessed with MMM's, years ago I started a card file with listings for all such heroes I could find - those that weren't in Rovin. So as Roy suggests, I had to cobble together a definition. If a character ticked 2 out of 3 of the following boxes, I included them.
1/ Super power, gadget, weapon
2/ Secret i/d
3/ mask and/or costume and/or symbol on costume (preferably 2 out of 3 again)
3a/ my decision, 'cos it's ma ba'.
Some of you wont like this definition but it suits my purposes.
On further thought, we tend, or at least my associates in fandom here do, to use the term superhero to cover all MMM's or costumed heroes, a bit lazy but shorthand. But not all MMM's are superheroes in the strict sense of the word but everyone who dresses up in a costume or dons a mask to hide another identity is a Masked Mystery (Wo)Man. I think.
So Zorro, Lone Ranger (Who was that masked Man?), Miss Masque, Aussie Shadow, Gunmaster, Maskar
http://www.brunoruffo.it/Maskarb.gifAsso di Picche
http://www.archivespratt.net/asdepique.htmHourman, are all MMM's but only one has super powers. I don't include Holmes, Blake etc. as they belong in the Great Detectives category and don't really fit the definition. Heroes certainly but without the other baggage. And some characters could, of course, appear in both categories i.e. Phantom Detective and Batman.
At this point you're probably thinking, "How sad" and you may well be right but it keeps me out of trouble and I've stumbled upon characters even some of the experts here will never have heard of.
And before anyone asks, not Doc. Savage. A friend does the same sort of exercise and Doc. is in his Great Adventurers classification.
This is just fun for me. Please don't take it too seriously and if you want info. on characters such as TNT Tom, Phantom Raider, Billy the Cat, Thunderbolt (no, not that one), Super Boy (no, not that one), Zakimort, Phantom Knight, Yelmo Negro or umpteen others, as they say in Asda, "Happy to Help"