means A straight, slender bar or stick of wood, metal, or another material, used for measuring, supporting, fishing, conducting electricity, or any of a dozen other jobs — and, in anatomy, one of the slim light-sensitive cells in the eye that handle vision in dim light.

from From Old English 'rodd,' a slender shoot or stick, with relatives across the Germanic family — likely a cousin of Old Norse 'rudda,' a club. The word stayed close to its literal self for centuries, then branched out: a 'rod' became a unit of length (about 5.5 yards, the old surveyor's measuring pole), a symbol of authority and punishment (the 'rod' that spoils no child), and eventually the eye's dim-light cells, named in the 19th century for their literal rod-like shape under the microscope.