A fall-off is a drop in a level, an amount, or a number — sales, interest, quality, visitors. Newspapers and business people like it: “a sharp fall-off in tourism.” It names the drop itself, not a person hitting the ground. Though actually, in everyday talk people more often say a drop or a fall; fall-off sounds a little like a report. You may also see falloff, one word; the hyphen is common in learner dictionaries. It is not a huge, technical word, but it does sit in writing more than in chat.

下滑、減少（業績、買氣、人數往下掉；新聞常用「下滑」）