1. A very steep or perpendicular descent
2. A marked dwindling or decline
3. The act or an instance of making a usually brief deposit or delivery
4. Something that has been dropped off is a drop-off. The place where something or someone is dropped off is a drop-off point, drop off window, and soon.
5. A decline in a business's sales, in the occurrence of an event, or in the interest some people have in something, is a drop-off.
6. A steep decline in the level of the ground is a drop-off.