A shadow is a dark shape, e.g. on the ground or a wall, caused by an object (or person, etc.) blocking light. Like a silhouette, the shape of the shadow is a two-dimensional projection of the object, but:
For non-point sources of light, the shadow is divided into the umbra and penumbra. The wider the light source, the more blurred the shadow.
If there are multiple light sources there are multiple shadows, with overlapping parts darker. For a person or object touching the surface, like a person standing on the ground, or a pole in the ground, these converge at the point of touch.
If white light is produced by separate colored light sources, the shadows are colored.
A shadow cast by the Earth on the Moon is a lunar eclipse. Conversely, a shadow cast by the Moon on the Earth is a solar eclipse.
Fiction: In Peter Pan the main character loses his shadow: it snaps off when he leaps out of the window, which is slammed closed behind him. It is put in a drawer and later sewed back on by Wendy.
See also shade.
More generally the term shadow is also used with regard to other things than light, for example rain: a rain shadow is a dry area, which, with respect to the prevailing wind direction, is beyond a mountain range; it is dry because air masses lose part of their water when they move over these mountains.
In Jungian psychology, the shadow is a part of the unconscious mind which is mysterious and often disagreeable to the conscious mind, but which is also relatively close to the conscious mind. It may be (in part) one's original self, which is superseded during early childhood by the conscious mind; afterwards it comes to contain thoughts that are repressed by the conscious mind. The shadow is instinctive and irrational, but is not necessarily evil even when it might appear to be so. It can be both ruthless in conflict and empathatic in friendship. It is important as a source of hunches, for understanding of one's own more inexplicable actions and attitudes (and of others' reactions), and for learning how to cope with the more problematic or troubling aspects of one's personality.
The shadow may appear in dreams and visions in various forms, often as a feared or despised person or being, and may act either as an adversary or as a friend. It typically has the same apparent gender as one's persona. It is possible that it might tend to appear with dark skin to a person of any race, since it represents an old ancestral aspect of the mind. The shadow's appearance and role depend greatly on individual idiosyncrasies, because the shadow develops in the individual's mind rather than simply being inherited in the collective unconscious.
Interactions with the shadow in dreams may shed light on one's state of mind. A disagreement with the shadow may indicate that one is coping with conflicting desires or intentions. Friendship with a despised shadow may mean that one has an unacknowledged resemblance to whatever one hates about that character. These examples refer to just two of many possible roles that the shadow may adopt, and are not general guides to interpretation. Also, it can be difficult to identify characters in dreams, so that a character who seems at first to be a shadow might represent some other complex instead.
According to Jung, the shadow sometimes takes over a person's actions, especially when the conscious mind is shocked, confused, or paralyzed by indecision.
The shadow might be the basis of the rank of Corax (raven) in the ancient religion of Mithraism.
In politics, the word "shadow" can refer to an opposition group, such as the Shadow Cabinet, or an alternative, secret shadow government.
The Shadows was a guitar-based rock band led by Hank Marvin, formed in the early 1960s. For many years they toured with Cliff Richard as his backing group.
In the television science fiction series Babylon 5 a war is fought against an ancient extraterrestrial race called the Shadows (or Shadowmen according to the original scripts).