C 187 refers to the California Penal Code section for Murder in the first degree. All other human killings are some form of manslaughter. The "C" in the term "C 187" refers to the word "Code" in "California Penal Code".
John Jones, you have been charged with Section 187 of the California Penal Code, Murder in the first degree. How do you plead?
Yes, Mr. Jones, you.
To C 187? Yeah, I done it! So what?
Take him away!