Everybody who follows world affairs knows what a treaty is. In Washington it has a special meaning. It is a document which has been negotiated, signed by the president, and ratified by a vote of two-thirds of the Senate.
A commitment so voted and so ratified is binding not only on the administration in office at the time, but on succeeding administrations.
An ''executive agreement'' is something less than a treaty because it has not been sanctified by that two-thirds of the Senate. But most executive agreements used in Washington have been authorized by a majority vote in the Congress first and then executed and signed by the president. Trade and tariff agreements are usually of this kind. They tend to be binding on present and future.
But what is a ''memorandum of understanding'' between one cabinet officer and his opposite number from some other government? It has not been ratified by two-thirds of the Senate or sanctioned by a majority vote of Congress. It is not signed by the president. In the immediate case of the ''memorandum of understanding'' signed last week by United States Secretary of Defense Caspar Weinberger and Israeli Minister of Defense Ariel Sharon the document was not authorized by any vote in the Congress, or signed at the White House, or even signed by the secretary of state. It was only what it says it is on the surface, a memo on an understanding reached by two defense ministers after talks between the two.
Therefore its validity, its effectiveness, and its value are like a big question mark. What does it really mean? How binding is it? Does it bind President Reagan and Secretary of State Alexander Haig, or only the Defense Department and then only so long as Caspar Weinberger is running things at the Penta-gon?
There is no clear answer to any of these questions. A document of this kind is open to a wide range of interpretation. And that is precisely why it has pleased neither the Arabs nor the Israelis. Each assumes the worst, from opposite points of view.
To the Arabs, it represented an American sellout to Israel. When Philip Habib , Washington's special ambassador in the Middle East, called on Syria's foreign minister Abdel Halim Khaddam he was told that ''the United States no longer has the right to play any mediation or arbitration role in the Arab-Israeli conflict because you have become a direct party in this conflict.''
But in the Knesset the same day opposition speakers pounced on the signing of the agreement as either being useless, or as committing Israel ''to operate against a world power, the Soviet Union, while there are questions about American promises.''
There were indeed questions about the promises under the ''memorandum.'' They are wide open. They promise consultations between the two defense departments, which go on all the time, anyway. They talk about possible advance ''prepositioning'' in Israel of American equipment presumably for future use of the US Rapid Deployment Force. But the type of such equipment is left unspecified and wide open. The only specific military commitment by the US is to hold ''joint US-Israeli exercises in the eastern Mediterranean Sea.''
The memo does say that the purpose of these vague proposals is ''to deter all threats from the Soviet Union to the region.''
So in the agreement Israel took up an anti-Soviet posture while obtaining no promise of a single extra American tank or gun or bullet in Israel.
But it had the effect on the Arabs of pushing them farther away from the US. Thus it makes more difficult and uncertain the possibility of engaging even the most moderate of Arab states in the ''strategic consensus'' which remains President Reagan's theoretical method of protecting the Middle East from possible Soviet intrusion.
The effect in Washington is new uncertainty about the attitude of the Reagan administration toward the vital central issue in the Middle East, which is the future of the Arabs who inhabit the ''occupied territories.''
Does it favor that true ''autonomy'' for the Arabs of West Bank and Gaza which the Camp David accord requires by the time Israel is supposed to withdraw from the final segment of the Sinai peninsula?
The deadline is only five months away.
President Reagan can have Arab friendship and cooperation if he will require real autonomy for the Arabs. But Prime Minister Begin is holding out for a settlement under which Israel will claim sovereignty over the occupied territories and retain military and police control and also control over land and water distribution.
The ''memorandum of understanding'' is typical of a government which cannot decide whether to allow Israel to annex all of Palestine, or require it to set free the Arabs of the occupied territories. No one can be sure which way it will resolve this central and difficult issue. The memo proves that uncertainty.