Ductility is the physical property of being capable of sustaining large plastic deformations without fracture (in metals, such as being drawn into a wire). It is characterized by the material flowing under shear stress.

A ductile material is any material that yields under shear stress (as opposed to brittle fracture, which yields under normal stress). Gold, copper, and aluminium are highly ductile metals.

Ductility is related to malleability.

In Earth science, the brittle-ductile transition zone is a zone at an approximate depth of 10 km in the Earth, at which rock becomes less likely to fracture, and more likely to deform ductilely. In glacial ice this zone is at approximately 30 metres depth. It is not impossible for material above a brittle-ductile transition zone to deform ductilely, nor for material below to deform brittly. The zone exists because as depth increases, confining pressure increases, and brittle strength increases with confining pressure but ductile strength remains constant. The transition zone occurs at the point where brittle strength exceeds ductile strength.

In physics/materials science the ductile-brittle transition temperature (DBTT) represents the point at which a BCC or HCP materials (e.g., a metal) fracture energy passes below a pre-determined point (for steels typically 40J (J.Vernon, Introduction to engineering Materials) for a standard Charpy Impact test). The transition from ductile to brittle behavious as the temperature is decreased is normally marked. DBTT can also be influenced by external factors such as neutron irradiation which leads to an increase in internal lattice defects and a corresponding decrease in ductilility/increase in DBTT.

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