A wonder nine
is a term used to refer to 9mm semiautomatic pistols
that have a significantly high capacity.
Glock 17 (17 rounds), Glock 19 (15 rounds), Beretta 92F (15 rounds), and the USP9 (15 rounds) are just a few examples of the more common wonder nines.
The wonder probably comes from the fact that you can shoot more rounds before reloading than an adversary who might be packing a revolver (6 rounds) or even a Colt 1911 (7 rounds).
The first gun to qualify as a wonder nine probably would be the Browning Hi Power, back in 1935 its 13 round capacity magazine was the highest capacity in a sidearm.
This is really of no significance if you consider that today you can now buy 30 or 33 round magazines for most auto pistols but if you consider that WWII was won by men wielding semiautomatic 8 round capacity rifles, you get an appreciation for the value of high capacity. On the other hand, knowing you have a lot of ammo can lead to the spray and pray technique of shooting.