Major Wound (or MW) is defined in the Basic Set
B419 "crippling wound is also a major wound"
B420 defines it as any hit exceeding 1/2 HP, or any lesser hit which cripples a body part.
It has a chance of causing Knockdown (resisted by a HT roll) and sometimes there are penalties for that roll based on certain locations.
quotes[]
Kromm in 2011 at http://forums.sjgames.com/showpost.php?p=1178365&postcount=4 regarding Critical Hits
- "Major wound" is an injury result like shock, unconsciousness, and death. It isn't a specific injury. Injury over HP/2 causes this result; crippling a body part causes this result; the right critical hit causes this result; and many other things can cause a major wound. The effect is a HT roll to avoid stun and knockdown. Never invert cause and effect here . . . receiving a major wound due to a body blow that doesn't inflict injury over HP/2 or a limb hit that doesn't cripple, probably due to a critical hit, doesn't mean that you advance the injury counter or inflict crippling, only that you have to make a HT roll. Grievous injuries are the most common cause of major wounds, but a major wound result can also occur directly – again, usually due to a critical hit.