That thin line between what is and is not news was on display last week at a breakfast meeting President Reagan had with some 45 of Washington's senior journalists.

For economic and financial writers there was no ambiguity. The president said interest rates should be lowered and the banking industry, not the Federal Reserve System, should see to it. That was reported uniformly.

For the rest of this extraordinary hour-long press conference without television, the main story became a question of what paper do you read.

The Los Angeles Times featured Mr. Reagan's allowing that there's "logic" to raising the Social Security retirement age beyond 65, giving second place to his comment that a buildup of Soviet missiles in Syria is "alarming."

The Baltimore Sun put the emphasis on presidential readiness to expand the multinational peace force in Lebanon while calling again for the withdrawal of Israeli, Syrian and PLO military. The Sun also reported Mr. Reagan as taking "a step back" from what he told an American Legion conference about arms control a day earlier.

The Christian Science Monitor wrote a catchall lead: "Firmness on the Middle East. Flexibility on arms control. Coziness about his political future. Defense of his record on environmental protection." Some of these subjects were in other stories.

The Post saw the main news in Mr. Reagan's saying the Soviet Union is likely to negotiate more seriously on nuclear weapons in Europe as the deployment of new American missiles draws near. The Boston Globe carried a parallel story.

The New York Times and New York Daily News led their papers with the president's comment that "something in the nature of a homeland" for Palestinians was necessary for Middle East peace. Although he added that he was not suggesting "creating a nation," it was the first time Mr. Reagan used the charged word, and it was striking that other stories, including The Post's, failed to mention it. Inevitably, there's been debate around newsrooms whether the more fallible news judgment was in overplaying or overlooking it.

White House correspondent Lou Cannon, relying, he said, on a colleague's notes from breakfast, a transcript and subsequent questioning of White House officials, included in his story a presidential restatement of concern for the security of Israel's northern border. That, incidentally, was drawn from the same response in which "homeland" was used.

Meanwhile, deputy national editor James R. Dickenson, who attended the breakfast, wrote a piece at the request of editors of The International Herald Tribune. It was an almost mirror image of Mr. Cannon's story, reversing the emphasis on arms control and Middle East, also without reference to "homeland."

No policy difference should be inferred from the president's remark, the White House said. Describing Post reporting on the press meeting and American Legion speech as "right on the money," a National Security Council official said Mr. Reagan "was very clear" and consistent with what he said about Palestinians in his Middle East proposals last September. That's where he said "the departure of the Palestinians from Beirut dramatizes more than ever (their) homelessness." Question: does he use "homelessness" there aware of the special force of "homeland," or use the latter as naturally derivative?

Predictably there was reaction in Jerusalem. "It is not by accident that this item does not appear in the Camp David Accords," said Israeli Foreign Minister Yitzhak Shamir, while other Israeli sources were reported by AP to believe Mr. Reagan "intended to encourage Jordan's King Hussein to join the peace process--all of which is more temperate than the response Jimmy Carter got for using the same word in 1977. The AP account placed The Post in the position of backing into the story by reporting for the first time what stimulated the reaction.

If Mr. Reagan's invocation of "homeland"--even "something in the nature of"--didn't merit reporting by all news desks, not as bulletin material, it should have. Now editors will watch to see whether officials stow the language rather than rationalize it as non-news, or make it commonplace, in which case the result could be the same.