名詞 drop-off:

1. a noticeable deterioration in performance or quality
   同義詞：slump, slack, falloff, falling off
2. a steep high face of rock
   同義詞：cliff, drop
3. a change downward
   同義詞：decrease, lessening

動詞 drop-off:

1. fall or diminish
   同義詞：drop off
2. change from a waking to a sleeping state
   同義詞：fall asleep, dope off, flake out, drift off, nod off, drop off, doze off, drowse off
3. leave or unload, especially of passengers or cargo;
   同義詞：drop, drop off, set down, put down, unload, discharge
4. retreat
   同義詞：fall back, lose, drop off, fall behind, recede
5. get worse
   同義詞：slip, drop off, drop away, fall away

drop-off

n.（名詞）
A steep or abrupt downward slope.
陡坡：陡的下坡
A noticeable decrease:
明顯地減少：
例句：
a drop-off in attendance.
出席人數(shù)的明顯減少