From The Collaborative International Dictionary of English v.0.48 [gcide]:
drop-off \drop-off\ n.
1. a noticeable decline in performance; as, a drop-off in
attendance. [WordNet sense 1]
Syn: slump, falloff, falling off.
[WordNet 1.5]
2. a steep high face of rock.
Syn: cliff. [WordNet sense 2]
[WordNet 1.5]
3. a change downward; as, there was a sharp drop-off in
sales. [WordNet sense 3]
Syn: decrease, lessening.
[WordNet 1.5]