DOUBLE JEOPARDY is one of those legal phrases that made its way into the common language so long ago it seems strange that the justices of the Supreme Court are still arguing about its meaning. Everyone else knows or, at least, has a feeling for what it means. Yet the justices split, 5 to 4, on Monday when they tried to define it.

Five of them said a person is not placed in double jeopardy when an appellate court reviews and increases the prison sentence imposed weeks or months earlier by a trial judge. Four of them said that arrangement clearly exposes the convicted criminal to the kind of double joepardy the Constitution forbids. The question is a close, and somewhat esoteric, one in legal and historical terms. The arguments each side adopted have been made over and over again during the last decade by less important definers of legal phrases and will, no doubt, go on being made in the law reviews and elsewhere.

But the issue is not so close in practical terms. By defining double joepardy as it did, the court's majority cleared the way for some long overdue and badly needed changes in the way final decisions are reached on how long criminals stay in prison.

The basic problem in the existing sentencing system is, as Judge Marvin E. Frankel observed some years ago, "the unbridled power" of those who impose sentences to be "arbitrary and discriminatory." This means that different judges can -- and do -- impose quite different sentences on similar defendants for almost identical crimes. The potential of this practice for claims of injustice, inmate unrest and shopping for "soft" judges is obvious. So is the need for parole boards to try to mitigate those problems by slicing time (sometimes wisely and sometimes not) from the longer sentences.

Civil libertarians have long argued that to protect individuals against the more vindicitve judges, appellate courts need the power to review and reduce sentences. While that change would be worthwhile -- it would eliminate the need for parole boards to consider the uneveness and injustice of discordant sentences -- it would leave the other half of the problem unsolved. What should be done about sentences that discriminate in favor of a particular criminal instead of against him?

If, as the four dissenters on the Supreme Court said, the imposition of a sentence ends a criminal case as definitively as does an acquittal, nothing could be done about that problem. A review of a sentence that led to an increase would be, ipso facto, double jeoprady. If, as the majority ruled, sentencing can be more than a one-step affair, the problem can be addressed. Double joepardy, after all, is putting an individual's life or liberty or property at risk more than once for the same offense.

The meaning of those two words, we suppose, comes down to how you feel about the concept they embody. There is some unfairness in a system that permits a person to walk out of court thinking he is off to serve three years in prison only to learn a few months later that the term will actually be twice that long. There is also some unfairness in a system under which a judge can go easy, for bad as well as good reasons, on particular defendants. These negative features seem to us to outweigh the positive ones -- by just about the margin by which the point of view prevailed among the justices.