A D battery (D cell or IEC R20) is a size of dry cell.
A D cell is cylindrical with an electrical contact at each end; the positive end has a nub or bump.
D cells are typically used in high current drain applications, such as in large flashlights, radio receivers and transmitters and other devices that require an extended running time
 

Common use names 

• 4020 (VERTA)

• MN1300 (U.S.A) DURACELL

• AM- 1 (JIS)