A narrow opening between two parts of a material that have split or become broken but may not be separated from each other. Usually used to describe breaks in more solid materials, such as stone, ceramic and wood. In plastics, refers to splits that follow the grain or direction of manufacture. 

A network of cracks has developed in the paint of this oil on board. Photo courtesy of Alice Cannon. 

A crack has developed across the base of this lacquer box. Photo courtesy of Alice Cannon.