What is the one-word term for a decrease in economic activity that could last for months or longer

Recession.

Explanation:

Recession: It is defined as a gradual decline in economic activity as consumer and business spend a lesser amount of money in the market, which leads to a decline in the gross domestic product of the nation. If there is a decline in GDP for consecutive two quarters then it is an indicator that the economy is heading toward recession. It is also considered a business cycle contraction.