A determinate sentence is a jail or prison sentence that has a defined length and can't be changed by a parole board or other agency. For example, a sentence of six months in the county jail is determinate, because the prisoner will spend six months behind bars (minus time off for good behavior, work-release, or other alternatives to in-custody time, when applicable).
By contrast, an indeterminate sentence is one that consists of a range of years—for example, "20 years to life." With an indeterminate sentence, there is always a minimum term (which, again, may be lessened by credits), but the release date, if any, is uncertain. It will be determined by a parole board when it periodically reviews the case. The state parole board holds hearings that determine when, during the range of the sentence, the convicted person will be eligible for parole.
Indeterminate sentences may be handed down for felony convictions, where punishment includes incarceration in a state prison. They are not generally used when the crime is less serious.
Indeterminate sentencing used to be the rule in every state and for the federal courts as well. Crimes usually carried a maximum sentence, but judges were free to choose among various options—imprisonment, probation, and fines. Parole boards decided on release dates.
The principle behind indeterminate sentences is the hope that prison will rehabilitate some offenders, and that different people respond very differently to punishment. Prison officials generally like indeterminate sentencing because the prospect of earlier release gives prisoners an incentive to behave while incarcerated.
With indeterminate sentencing, the goal is that offenders who show the most progress will be paroled closer to the minimum term than those who do not. The decision takes into account the individual offender's crime (including mitigating or aggravating circumstances), criminal history, conduct while in prison, and efforts toward rehabilitation. The victims of the offender's crime may also submit statements. There is, at least in theory, a careful and specific evaluation before the offender is released back to the community.
The problem with indeterminate sentencing, according to its critics, is that it puts too much power into the hands of the parole board, leading to arbitrary and discriminatory results. They charge that too often, minorities and prisoners without connections receive overly harsh decisions from parole boards, while less deserving offenders are released early.
Determinate sentencing began to spread widely during the 1970s and 1980s and is now the rule in many states. It's often seen as a "tough on crime" system because of its mandatory minimum sentences. Its proponents claim that it also leads to greater fairness, because when the legislature sets a determinate sentence and judges have little discretion, people who commit very similar crimes receive very similar sentences.
Indeterminate sentencing, however, is making a comeback in a time of prison overcrowding and lower crime rates. More room for judicial or parole board discretion is being let back into the sentencing systems of many states, especially for drug crimes, where rehabilitation is seen as a reasonable and attainable outcome for many convicted offenders.