What exactly is "involuntary manslaughter" in the Alec Baldwin case?

Involuntary manslaughter is thus very different from the voluntary; the similarities are just that it’s a homicide but not murder. One branch of it (“manslaughter committed in the commission of an unlawful act not amounting to felony”) is the so-called “manslaughter-misdemeanor” rule, an analog to the “felony-murder” rule. The second branch involves, basically, causing death through negligence.

But not just any old negligence, of the sort that we’re familiar with from civil cases. Rather, it has to be “criminal negligence,” which is defined in New Mexico as “willful disregard of the rights or safety of others”—what some other states might call “recklessness[.]” …

Say, then, that the prosecution can show that Baldwin pointed the gun at Hutchins and pulled the trigger, but carelessly believed (without checking this for himself) that it was unloaded.

It wouldn’t be enough to show that Baldwin was careless, negligent, or lacked due caution in the ordinary sense of the word. The prosecution would have to prove, beyond a reasonable doubt, that he was subjectively aware of the danger: that he actually thought about the possibility that the gun might be loaded, and proceeded to point it and pull the trigger despite that. That’s much harder than just to show carelessness, or even gross carelessness, though of course much depends on what evidence the prosecution has gathered.