Definition of Depression (in the context of the term to describe a movement of the body):
Depression is the downward movement of structures of the body, e.g. depression of a shoulder joint lowers the corresponding arm vertically downwards.
The movement of depression is the opposite of the movement of elevation.
The word depression may be used to describe a movement so it could be used in a phrase such as "depression of the shoulder is when the shoulder joint, together with the arm attached to it, moves downwards".
A depressor is a muscle that can act to cause a depression movement. E.g. the facial muscles the depressor labii inferiori draw downwards the skin tissues at the outer edges below the lower-lip to form a "sad" shape, i.e. the opposite position of the lower lip to that seen when the person is smiling.
Example of Depression (movement):
Depression of the shoulder blades (scapulae):
The term "scapular depression" is used to refer to the downward movement of the shoulders, e.g. when using the muscles of the shoulder girdle to lift weights using a pulley-system (weights training machine) as shown in the following video clip.