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.

1. To fall or diminish
2. When a business's sales, the occurrence of some event, or the interest some people have in something declines, it drops off.
3. When the level of the ground declines steeply, it drops off.

When you take things or people to another place and leave them there, you drop them off.