isn't it called manslaughter if it's 'spur of the moment'?
Manslaughter, inthe U.S., is generally when you kill someone without intending to kill them, but you acted negligently (generally, "with gross negligence"). For instance, if you drive drunk and cause an accident in which someone dies.
There are also different degrees of murder, which may be called by different terms in different jurisdictions, for example: felony murder (killing someone while engaged in another crime which is already a felony, for example, killing someone while robbing a bank), first degree, or premeditated murder; second degree murder, which is usually another term for a murder that wasn't premeditated.
What constitutes premeditation can really vary from state to state. In criminal law, we read a California appellate court case about a lodger who had tried to rape the teenage daughter of the house when she came home from school. She fought him, and he ended up stabbing her several dozen times all over the house, while she tried to get away. The court in that case found that the evidence supported lack of premeditation, that he had intended only to rape her, not kill her, and the situation had just gotten out of hand. They pointed to the bloody mess all over the house as evidence that he hadn't acted with premeditation, because if he had, he would have been more careful.
In New York, around the same time, an appellate court found that the evidence supported a finding of premeditation in the case of a woman who shot her husband during the course of an argument - she had to reach into the drawer of a bedside table to pull out the gun - the couple of seconds it took her to do that was sufficient time for her to "premeditate" the killing.
I've always remembered those two cases, because I thought they were so ****ed up.