An act that constitutes the crime of first-degree murder.
The criminal offence of murder in a form considered by law to be the most serious; the precise distinction may vary by jurisdiction but typically involves premeditation, or the choice of victim or other circumstances being regarded as especially blameworthy.
A person who commits multiple (more than two) murders, especially similar ones with no obvious motive over a period of time with a "cooling-off" period between each murder.