"Knockdown" power is a term used by John "Pondoro" Taylor years ago for qualifying Big Game rifle calibers in Africa. "Stopping" power is a far more modern term and is indeed relevant as some cartridge/bullet combos have shown to be far better at STOPPING an adversary with a good hit than others. By using the term "stopping" I mean forcing someone to stop what they are doing (usually trying to hurt/kill you). This does not imply directly killing the individual, however the nature of matter sometimes entails as such.

The rounds I would put in this category are the 9mm 115 gr. +p+ (such as the legendary Fed. BPLE & Win. as well), 125 gr. JHP .357 Mag. & .357 SIG and finally the 155 gr. Rem. .40 S&W JHP as loaded for the BP. These rounds have stellar histories that are above other contemporary rounds for their respective eras.

Just my 2 cents (worth 0.0015 under Obama).