What is a defect? A missing atom here, an extra atom there, an impurity, a stacking fault, a dislocation? Traditionally defects are considered exceptions to perfection—individual, relatively easily identifiable abnormalities in an otherwise pristine crystal structure.

However, each type of defect can star in opposing roles, as friends to some and foes to others. Depending on who you ask, one materials scientist might speak highly of a particular defect and its benefits, while another may avoid it at all costs—such as dislocations in metals versus semiconductors.