Shadow is diminution of light.
Darkness is absence of light.
Shadow is divided into two kinds, of which the first is called primary shadow, the second is derived shadow. The primary shadow is always the basis of the derived shadow.
The edges of the derived shadow are straight lines.
The darkness of the derived shadow diminishes in proportion as it is remote from the primary shadow.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.