It's called the OKC3S
(OKC stands for Ontario Knife Company and the 3S means 3 serrations)
It's also called the USMC Multi-Purpose Bayonet. Replaced the M7 bayonet in 2002. It can double as a fighting knife. The length of the blade is 8". Damn thing costs nearly $150. It's on backorder right now. Not that I plan on using it, but as a collection (not a knife collection, but a collection of things I like). Plan on taking basic blade lessons so I can at least use it for self defense in places where my handgun would be illegal to carry (as in California, lots of places) without cutting myself up. However, I plan never to use it in real life other than extreme utility purposes, self-defense training, or as a survival knife if I ever get lost hiking.