What does "depression" mean?

Additional senses

1. 2.A falling in of the surface; a sinking below its true place; a cavity or hollow; as, roughness consists in little protuberances and depressions.
2. 3.Humiliation; abasement, as of pride.
3. 4.Dejection; despondency; lowness. In a great depression of spirit. Baker.
4. 5.Diminution, as of trade, etc.; inactivity; dullness.
5. 6.(Astron.) The angular distance of a celestial object below the horizon.
6. 7.(Math.) The operation of reducing to a lower degree; -- said of equations.
7. 8.(Surg.) A method of operating for cataract; couching. See Couch, v. t., 8. Angle of depression (Geod.), one which a descending line makes with a horizontal plane. -- Depression of the dewpoint (Meteor.), the number of degreees that the dew-point is lower than the actual temperature of the atmosphere. -- Depression of the pole, its apparent sinking, as the spectator goes toward the equator. -- Depression of the visible horizon. (Astron.) Same as Dip of the horizon, under Dip.