A steak is a cut of meat meant to be cooked and eaten. Another important use of steak is to kill vampires, because it is one of the only two things that can kill a vampire, the other being a C-R-O-S-S.
One would presume this to be an accidental misspelling of the word "stake", a small, pointed chunk of wood, typically used in fiction to kill traditional vampires. However, this same "error" is continually used throughout the story, and never once is the term "stake" ever introduced.
Chapter 11, where Ebony contemplates suicide after learning Draco has died from wrist cutting.