“Life imprisonment without parole” can mean precisely that – that, barring a positive decision on the part of the Governor (or President if a Federal prisoner) to commute the sentence, the person is never eligible for release, on any grounds.
However, it can also mean something else. And the significance here lies in the idea of “indeterminate sentences.” These are very useful things for the court system in dealing with the proper sentencing for severe felons.
A typical jail sentence is determinate – the judge says, “Thirty days.” Of course, what most people don’t realize is that that means “thirty days, subject to good behavior time.” If a prisoner stays out of trouble for the majority of his sentence, he’s entitled to be released when the majority of his sentence has been served. At one time, New York’s laws provided for release after 5/6 of a sentence, subject to the Sheriff or his Jailer taking away “good behavior time” for misbehavior while in jail. So the guy with a 30 day sentence would be eligible for release in 25 days unless he forfeited good behavior time.
In an indeterminate sentence, on the other hand, the judge sets a maximum and a minimum sentence, the minimum typically a quarter to a half of the maximum, and very often a third. So Joe the Burglar gets sent up for a sentence of four to twelve years. What this means, effectively, is that he must serve a minimum of four years before being eligible for parole. Parole is discretionary at the judgment of the Parole Board for anything thereafter. But there is also a kicker: prisons also have “good behavior time” – and once someone has served what the law provides, he’s entitled for release on parole if he chooses to take it. (Some prisoners actually prefer to serve out their sentences and leave prison a free man, with no need to account to a parole officer or live under the conditions of parole.) In New York, at the time I learned this stuff, that was at the point 2/3 of one’s maximum sentence had been served. So Joe the Burglar, having served four years, becomes eligible for a parole hearing. He is denied parole at that hearing, and every six months thereafter gets a hearing and is still denied. He’s now served eight years, 2/3 of his four-to-twelve. At this point the onus flips: rather than him having to prove to the Parole Board how he has reformed and deserves parole, the burden is on the other side, to prove why he should be denied parole.
In some states, “life imprisonment” with no modifiers technically stands for “an indeterminate sentence with a minimum of seven years and a maximum of the rest of his natural life.” He thus becomes entitled to a parole hearing (not necessarily to parole) after serving seven years. And there is some point, which varies by state, where good behavior entitles him to parole, barring the specific grounds for denying it to him. (Let’s arbitrarily say 20 years for purposes of explaining this.) In other words, he must serve a minimum of seven years, may be granted parole at the discretion of the Parole Board at some point no less than seven and no more than 20 years after having been sentenced, and is eligible to be granted parole, absent a finding of good reason to deny him parole, after 20 years.
In those states, “life without parole” means that there is no point at which he becomes entitled to parole itself. But the parole board, in those states, still has discretion, if they are firmly convinced he has reformed.
In other states, of course, “life without parole” means precisely that. Barring clemency, there is no point at which he can be released on parole. But it’s important to realize that, e.g., Georgia’s seemingly extremly harsh sentencing laws actually mean, “serve a minimum sentence and show the parole board you have reformed,” not the 20 or 50 years they actually read as.