First-degree murder is a crime that is defined in a state's statute. However, the exact definition will vary by jurisdiction.
Some states will include the murder to be premeditated; others, however, have no such requirement. Many states disagree over the definition of premeditation, and as such, many states no longer include premeditation as a requirement of first-degree murder.
New York, possibly in observance of the confusion over "premeditation," has abandoned premeditation altogether as the test for first degree murder. Instead New York only charges the defendant with first degree murder when the defendant kills a police officer, kills a trial witness, or kills through terroristic acts.
Iowa uses an either-or approach. The state applies the New York rule for first degree murder, but it also allows courts to use a pre-meditation standard for first-degree murder if the court chooses to do so.
Ohio's first degree murder statute now uses “prior calculation and design” as the test for first degree murder instead of “pre-meditated.”
For more on first degree murder, see this New Mexico Law Review article, this University of Berkeley Law Review article, and this Northwestern University Journal of Criminal Law and Criminology article.