A shadow-like visual effect consisting of a character-shaped area of color offset from the character. The shape of a drop shadow is exactly the shape of the character; if it's angled or distorted, use shadow instead.
It's basically when it looks like there's a light source just above the character's head, projecting a shadow on an invisible wall just behind the character.
Examples
Non-examples
An actual shadow with a silhouette that is similar but not the exact same as the character
