(via dreamintuition.com)
In Jungian psychology, "shadow" or "shadow aspect" may refer to (1) an unconscious aspect of the personality which the conscious ego does not identify in itself.
Because one tends to reject or remain ignorant of the least desirable aspects of one's personality, the shadow is largely negative, or (2) the entirety of the unconscious, i.e., everything of which a person is not fully conscious. There are, however, positive aspects which may also remain hidden in one's shadow (especially in people with low self-esteem, anxieties, and false beliefs). To know yourself, you must accept your dark side. To deal with others' dark sides, you must also know your dark side.
According to Jung, the shadow, in being instinctive and irrational, is prone to psychological projection, in which a perceived personal inferiority is recognised as a perceived moral deficiency in someone else. These projections insulate and harm individuals by acting as a constantly thickening veil of illusion between the ego and the real world.
Jung also believed that "in spite of its function as a reservoir for human darkness—or perhaps because of this—the shadow is the seat of creativity"; so that for some, it may be, "the dark side of his being, his sinister shadow...represents the true spirit of life as against the arid scholar.”