Per Wikipedia:
http://en.wikipedia.org/wiki/ManslaughterInvoluntary manslaughter
Involuntary manslaughter, sometimes called criminally negligent homicide in the United States or culpable homicide in Scotland, occurs where there is no intention to kill or cause serious injury but death is due to recklessness or criminal negligence.
Criminal negligence
Negligence consists of conduct by an individual which is not reasonable -- that is, the individual did not act with the care and caution of a reasonable person in similar circumstances. This "reasonable person" is fictitious, of course, but reflects the standard of conduct which society wishes to impose. Violation of this standard may lead to civil liability for the consequences of the negligent behavior.
Negligence rises to the level of criminal negligence where the conduct reaches a higher degree of carelessness or inattention, perhaps to the point of indifference.
Recklessness
Recklessness or willful blindness is defined as a wanton disregard for the known dangers of a particular situation. An example of this would be throwing a brick off a bridge onto vehicular traffic below. There exists no intent to kill, consequently a resulting death may not be considered murder. However, the conduct is probably reckless, sometimes used interchangeably with criminally negligent, which may subject him to prosecution for involuntary manslaughter: the individual was aware of the risk of danger to others and willfuly disregarded it.
In many jurisdictions, such as in California, if the unintentional conduct amounts to such gross negligence as to amount to a willful or depraved indifference to human life, the mens rea may be considered to constitute malice. In such a case, the offense may be murder, often characterized as second degree murder.
In some jurisdictions, such as Victoria, recklessness is sufficient mens rea to justify a conviction for murder.