Involuntary manslaughter is a common law offense of killing either through recklessness, but without an extreme indifference to human life, or through gross negligence.
It requires either recklessness by consciously disregarding a substantial and unjustifiable risk to human life or gross negligence in failing to be aware of a substantial and unjustifiable risk to human life and committing an act exhibiting a callous disregard for human life.
One commits involuntary manslaughter when he kills another through a gross deviation from the standard of care that reasonable people would exercise in the situation.
It is more than tortuous negligence; it must be "so gross" as to be deserving of criminal punishment.
The Model Penal Code does not recognize gross negligence as a basis for involuntary manslaughter, but has a separate offense of negligent homicide.
When one kills another through an act that exhibits extreme indifference to human life, the difference between reckless murder and involuntary manslaughter is that reckless murder requires that the defendant consciously disregard the risk, while involuntary manslaughter only requires that he be grossly negligent by be being unaware of a substantial and unjustifiable risk to human life.
If one is aware of a substantial and unjustifiable risk to human life and disregards it anyway (recklessness), but his act does not exhibit extreme indifference to human life, he is guilty of involuntary manslaughter.
This is also manslaughter under the Model Penal Code.