Now, at last, it is becoming clearer what Jimmy Carter really hopes for in U.S. relations with the Soviet Union. It is not just a mere detente, but - much loftier - an entente. If the president can bring it off, more power to him, for the whole world would surely be better off.

Like detente itself, the word entente comes from the French, and stands in a general way for "understanding." In diplomacy, it is most commonly thought of as an agreement (usually not in writing) between two or several powers to work cooperatively together.

That is the positive kind of arrangement that Carter now seems to be calling for, despite the surface bluster of his most recent statements, which the politicians of both parties tend to dismiss as mostly for domestic consumption.

It is hard to remember when an American president has criticized the Soviet government more harshly or more directly than Carter did in his recent Naval Academy speech, yet throughout the address there are signs that, far from wanting to ditch detente, he is eager for a broader and more constructive agreement.

The president, like Zbigniew Brzezinski, his national security adviser, is plainly dissatisfied with the so-called "code of detente," as spelled out by the leaders who drafted it - former president Richard Nixon, former secretary of state Henry Kissinger and Soviet President Leonid Brezhnev.

Their narrow, if not negative, interpretation, which has prevailed since the agreement was adopted in Moscow in May 1972, severely limits the scope of the accord, but that is not at all what Carter and Brzezinski appear to have in mind for the future.

Attention has naturally centered on the president's melodramatic message to Moscow ("confrontation or cooperation"), but apparently it was not intended to be an ultimatum. Rather, it was meant as a way of emphasizing that detente under the Carter administration has got to be more "cooperative" than in the past if it is to survive. This is the way the president put it:

"To be stable, to be supported by the American people and to be a basic for widening the scope of cooperation, detente must be broadly defined and truly reciprocal."

Carter can hardly be faulted for urging a more productive relationship with the Soviet Union. The only trouble is that he has gone about it in a self-defeating way, for a new and better bilateral agreement surely cannot be achieved by a unilateral presidential declaration couched in the abusive terms that marked his Naval Academy speech.

Nixon and Kissinger effectively demonstrated that the key to useful agreements with Moscow is quiet diplomacy. The behind-the-scenes negotiations for detente were, in fact, so quiet that few even knew about them until the accord was almost an accomplished fact.

It takes two to make an agreement, so if Carter and Brzezinski want to replace detente with an entente, they had better take a tip from the Nixon-Kissinger team and open up private talks with the Kremlin.

It ought to be clear by now that the Russians, not unnaturally, resent having major new proposals sprung on them publicly without prior consultation. The Carter administration, for instance, got burned last year when it sent Secretary of State Cyrus Vance to Moscow with a surprise SALT package. The Soviets rejected it out of hand.

If Carter is serious about an entente, it is just possible that he might come close to getting one if he relies on the patient and tactful Vance to pursue it diplomatically. Fortunately, there are signs that the Russians - at least the Brezhnev bloc - also may be ready to move a step or two beyond the old detente.

Meanwhile, as an earnest of his sincerity, Carter can do two important, specific things guaranteed to produce a more cooperative climate in Moscow. The first thing is to make good on his assurance that he is not stalling on a new SALT agreement. SALT II is obviously the apple of Brezhnev's eye.

Second, President Carter can take the initiative in getting Congress to remove that other bone in the Soviet throat - the Jackson-Vanik amendment, which bans normal trade and credit relations with the Russians because of Soviet emigration policies.

That legislation, strongly opposed by Nixon and Kissinger, was the first direct slap at detente, for the regularizing of U.S.-Soviet trade was an integral part of the 1972 Moscow agreement.

Many of the legislators who voted for it now realize that it has been counter-productive, since it not only exacerbated U.S.-Soviet relations, but prompted Moscow to adopt an even more repressive emigration policy. It's a good moment for Carter to seek repeal of the amendment.