The Federal Reserve Board, deeply worried by a recession that has lasted longer than any other since before World War II, yesterday cut its discount rate from 9 to 8 1/2 percent.

It was the seventh half-point reduction since July for the rate charged by the Fed on loans made by its district banks to financial institutions.

Under present money market conditions, the level of the discount rate has become a key to the whole structure of short-term interest rates. With the latest cut, other rates, including commercial banks' 11 1/2 percent prime lending rate, likely will continue to come down. Long-term rates, such as home mortgage rates, may also.

After the Fed's late afternoon announcement, the dollar fell from its day's highs against foreign currencies, but dealers weren't certain about the long-term impact.

The Fed made it clear that it was pushing interest rates downward -- rather than ratifying a decline that had already occurred in the marketplace, its explanation for the other discount rate cuts of the past five months.

In a statement, it said it acted "in the light of current business conditions, strong competitive pressures on prices, and further moderation of cost increases, a slowing of private credit demands, and present indications of some tapering off in growth of the broader monetary aggregates."

In addition to economic considerations, the Federal Reserve also has been under growing political pressure to reduce interest rates. Numerous members of Congress, including Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) and Minority Leader Robert C. Byrd (D-W.Va.), have been warning the Fed that steps would be taken to limit its independence if rates don't continue to fall.

Some economists believe that only the United States can lead the way out of what has become a worldwide recession. The economists also say that only a recovery in the major industrial nations can ease the plight of some developing nations whose economies have been so damaged by collapsing markets for exports that they could default on their foreign loans and precipitate an international financial crisis.

Federal Reserve Chairman Paul A. Volcker is known to have been spending a substantial portion of his time in recent weeks trying to make sure no such crisis occurs.

The U.S. recession, which began in July, 1981, is now in its 17th month. The longest previous postwar recession lasted 16 months, from November, 1973, until March, 1975.

Moreover, since this current slump came only a year after the brief but sharp 1980 recession, it began with the economy still far from full employment. As a result, postwar records have been set for unemployment, which reached 10.8 percent last month with 12 million people jobless and another 6.5 million working part-time when they would prefer full-time work. Payroll employment fell by 550,000 jobs in October and November alone. The amount of idle factory production capacity is also at record levels.

Most economic forecasters believe that real gross national product will decline this quarter. However, Commerce Secretary Malcolm Baldrige said yesterday in a statement that he expects "little or no growth" in the GNP this quarter after adjustment for inflation. Real GNP also showed no change in the third quarter.

The forecasters in the administration and in the private sector continue to predict that a recovery will begin soon, just as they have for much of the past year, though they expect a much less rapid rebound than after previous recessions.

For instance, those at Morgan Guaranty Trust Co. believe that "lower interest rates, in time, should help nurture economic recovery," with real GNP growing at a 4 percent rate or better in the second half of 1983. At the same time, they caution, "that . . . does not promise any significant reduction of unemployment over the next year."

Any recovery is supposed to be sparked by an increase in consumer spending, and indeed retail sales rose 2.3 percent last month. But the gain came largely on the strength of a jump in auto sales promoted by below-market interest rates on new car loans. Similarly, a modest recovery is under way in the housing industry fueled by a drop in mortgage interest rates to 14 percent or lower.