The fact that Zimmerman fired the bullet that killed Martin was never in question, but the verdict means the six-person jury had reasonable doubt that the shooting amounted to a criminal act.
The verdict caps a case that has inflamed passions for well over a year, much of it focused on race and gun rights.
The six-person jury -- all women -- basically had three choices: to find Zimmerman guilty of second-degree murder; to find him guilty of the lesser charge of manslaughter; or to find him not guilty.
The jurors deliberated for 16½ hours total, including 13 on Saturday alone, before delivering its verdict.
Zimmerman smiled and then shook his lawyer's hands when he learned his fate. His parents, Robert and Gladys Zimmerman, were seated nearby, but Martin's parents were not in the courtroom.
Earlier in the day, the jury had asked the court for clarification on its instructions regarding manslaughter. The jury couldn't have even posed such a query a few days ago: Judge Nelson ruled Thursday, over the defense's vehement objection, to include manslaughter as an option for jurors, in addition to a second-degree murder charge.
The question -- the jurors' first since late Friday afternoon, when they requested an inventory of evidence -- was read out in court shortly before 6 p.m. Saturday. After a brief discussion between lawyers and the judge, the court recessed for about 40 minutes.
When it reconvened, shortly before the judge announced the jury had ordered dinner, prosecutors and defense lawyers agreed to a response to the jury's question -- basically asking for more detail. "The court cannot engage in general discussions but may be able to address a specific question regarding clarification of the instructions regarding manslaughter," their response to the jury says. "If you have a specific question, please submit it."
Manslaughter, under Florida law, is "the killing of a human being by the act ... of another, without lawful justification ... and in cases in which such killing shall not be excusable homicide or murder." It is a second-degree felony.
According to the jury instructions, Zimmerman could be convicted of manslaughter if jurors believe he "intentionally committed an act or acts that caused the death of Trayvon Martin."
"George Zimmerman cannot be guilty of manslaughter by committing a merely negligent act or if the killing was either justifiable or excusable homicide," the instructions add. "Each of us has a duty to act reasonably toward others. If there is a violation of that duty, without any conscious intention to harm, that violation is negligence."
If convicted of manslaughter, Zimmerman could face up to 30 years in prison. The jury, however, hasn't been told of possible sentence lengths for any charge.
To convict Zimmerman of second-degree murder, the jury would have to believe that "there was an unlawful killing of Trayvon Martin by an act imminently dangerous to another and demonstrating a depraved mind without regard for human life."
Such a killing would have to be "done from ill will, hatred, spite or an evil intent" and would be "of such a nature that the act itself indicates an indifference to human life."